Why can't we draw something well? This is an important question to ask. If you know the answer, you will be able to devise a plan of action that's best suited to your circumstances.
If somebody can't draw something, there's four main possibilities (maybe there's more? For now, here's four important ones):
1) they don't understand what something should look like. They have misconceptions about the appearance of whatever it is they're trying to draw. They take a look at their drawing and they're at a loss for where things best ought to be.
They might gain as much abstract knowledge as they want. This muscle is called this, this muscle is called that, this is above that. However, information such as this is is useless on its own.
If they don't have a solid grasp of how things should look; e.g., this thing should be
this much higher than that, this thing should be
this big, this thing should have
this kind of angle to it, their drawings will have problems that keep it from being the image it could be. They might get lucky in various places, but their drawings will overall have significant flaws. This knowledge is to a large degree non-verbal. The knowledge can be hinted at verbally, but its not the true form of the knowledge.
The thing is, what things should look like takes a lot of time to digest. What things should look like is not even apparent from looking at a single instance of it. Any thing has intricacies to how its surfaces curve or round...the geometry/structure/form...these are very important details. We must learn the 3D appearance of things.
In side profiles, we can see quite clearly the 3D appearance of things in terms of depth. We can see quite clearly the angle the neck is taking in depth, or how far away the tip of the nose is from the face.
However, we learn very little about a great deal, too; we gain, for example, almost nothing about how wide the nose is.
Anything that is difficult to draw is like this. One certain angle of it illuminates much, but for each angle much is also difficult and obscure. For such complex objects, it is highly beneficial to examine the object from all the main angles; front, side, 3/4 front, back, 3/4 back. One must internalize the various 3D characteristics of the object, not just do studies. It is visual information that you need to learn to rely on whenever you are drawing at all. Doing so grants an understanding of the
thing that cannot be paralleled through any other approach.
2) they lack the ability to represent form in space. Problems occur around concepts such as perspective, depth, form, etc. There are many ways to express what is at the core the same concept.
Examples (non-extensive):
- If they need a basic form to wrap around another, they are unable to draw it convincingly.
- If they need two forms in different points of space to have the same length, they are unable to keep the lengths consistent. For example, they produce a flaw in their drawing, where one arm is noticeably longer than the other.
- They are unable to maintain ~90 degree corners of architectural/non-organic objects when necessary
- If the thing has equally spaced sections, they are unable to maintain the spacing of the sections. For example, they are unable to make the space between railroad tracks seem equal as the tracks move back in space
- Inability to wrap patterns around basic forms. For example, eyes are (in some styles) drawn as flat symbols that wrap around the round surface of the face. Problems in wrapping that flat symbol on the face poses problems.
- Inability to maintain a sense of things being level in their drawing. Eyes are naturally level with each-other (in relation to the level of the head). In a neutral expression, a closed mouth also is level with the eyes. These things, therefore, need to appear level in this type of scenario.
- If two forms need to look like they are touching one another, they are unable to make it convincing.
3) Lack of mechanical ability
If you can't produce a straight line, then your drawing will be flawed whenever it ought to have straight lines. The same applies to drawing good ellipses/circles/curves. Accuracy is also an issue here. You may be able to draw the right line/ellipse, but you may lack the ability to place it where you want to place it.
Mechanical ability can be augmented with things such as rulers and stabilizers (traditional and digital). And yes, stabilizers exist in traditional as well. A mahl stick is a tool some painters use to rest their wrist on, thus stabilizing their hand.
4) Taking an ineffective approach
This one is subtle, but it is also where some people end up wasting a lot of time worrying. If you don't know what something should look like, any approach you take will not cover up for what you're missing.
But approach is still a possible good reason for why a good drawing can't be completed.
An example is drawing mecha or gundam. There's a lot of forms to keep track of. A good way to start would be drawing a base generic gundam/mech underneath, and then attaching the distinguishing elements on top. This way the core base of the mecha and the solidity of the pose are guaranteed from the beginning.
Conclusion:
Regardless of what it is you want to draw, there is the concept of "what it ought to look like." What something ought, or should, look like is to a degree up to you. That's stylistic choice. Realistic, semi-realistic, western, anime, super-deformed, chibi, etc.
However, every drawing has a direction where it wants to go most. For a good artist, the drawings they make exist in a direction relatively close to the direction they want it to go. For the frustrated artist, however, where they want to go is often out of sync with what their drawings are expressing.
I emphasize this because there is the potential for naming rules and problems to come across as academic. You know for yourself when you think a drawing doesn't look like how it should.
Some people may disagree on what things ought to look like. One person may really like overly long limbs. That's fine, and they probably can make their artwork work if everything else in their images works together with that idea. They know what their drawings should look like and they're going to end up making successful drawings because of that.
In truth, these four concepts are there for you to be used with
your artistic problems (not to judge
others' problems). So whenever you are having problems or are frustrated, hopefully you will be better able to figure out your situation and make progress where you want to make it by considering these 4 reasons.