I've talked before about "the wall", but not the different reasons for hitting it. Sometimes it's just not knowing your subject well enough, sometimes it's a change of direction, and sometimes it's losing interest in the subject or the painting itself.
A friend told me he loses interest in his art if it takes more than an hour or two to complete. For him, time is the wall. This is not a bad thing, it's good that he recognizes this aspect of how he has to work to maintain the quality he needs in his work. He has excellent drawing skills and a great sense of design. He limits his palette to just the essential colors, his mixing of colors is quite masterful. He has prepared himself quite well for doing his work in the amount of time he has before his interest fades. He avoids the wall by completing his painting quickly. He accepts whatever he ends up with, good or bad, he moves on to his next piece.
To overcome this short interest span he spends time drawing and painting the human figure from life. This practice helps with his quick study of all subjects he wishes to tackle. Though he may devote only an hour or two to doing a painting of an old truck, he has spent hours honing his drawing skills and his observational skills drawing from a live model. Each one of us artists must recognize our own pitfalls that build our walls and focus on ways to overcome them.
Time for some is that wall, but for some, it is not a factor at all. Whether we take a couple hours to complete a piece or a year to complete a piece each artist needs to work through their own wall or find ways around it. Subjects maybe incompatible with our medium resulting in a loss of vision for our finished piece. We fight and fight forcing ourselves to work though blindly not knowing what the real problem is. Are the colors we laid out right? How well do we know our medium? Are the brushes right for the ground we are working on?
The more we learn the less we seem to know.