Above: A 12 x 12 inches (approx.) charcoal sketch from life-drawing, on newsprint. The 18 refers to the 18th sketch from this day's session and the 20 referred to 20 minutes which is what I should have had, but this turned out to be a 15 minute sketch because I wasted time running around to check angles and finally did decide to move to another easel which I then had to set up. I like noting such things down. Click on the image to view an enlargement.
In the gesture drawing class I go to the last 40 minutes can be used as two 20 minute poses or one long pose. I tend to do two 20 minute poses: the first would be the full figure and the second is just the head and shoulders—I'm sorry I'm just really interested in noses, ears, and hair. At least I recognize that.
But there is gesture in anything and on this day there was a really cool thing going on in the model's neck so I wanted to get a bit of that. But time was running out and things really weren't working, which means I'd actually stopped looking. The instructor came over and pointed out that the shoulder on the right side of my sketch was too high so the angle under the chin wasn't working out.
With seconds to go I redrew that line lower (you can see the original line and the second line still ghosted in the sketch. Neither worked. With a couple seconds left before the model moved I hit the third line and it worked. (It goes down too low and at that first kink should be cut off because his chest is not going to be that wide, but there wasn't time to fix any of that.)
Sometimes our brains stare so hard they actually stop and we need someone to come along and jog us. I think this is one of the many benefits of going to a drawing class where you are given feedback.
If you don't have someone on hand who is trained to see (a significant other telling you the pear you just drew isn't round enough is of absolutely no value) you can help yourself in a couple ways. Step away, look away, for a moment and then look back, and try to see your sketch and subject in the same view at the same time. The "error" might jump out. Alternately, over time you can develop a checklist of sorts for when you are stuck. This list represents key points or angles to check, like the angle of the nose, the distance to the ear, etc. in a portrait. But that list can also contain the little checks you find yourself always doing because your habits get you into trouble. I tend to go wide on things so I constantly check widths. Knowing that, over time, my habit is a "little" less pronounced. So over time we can self correct as well as in the moment.
So get a second opinion either from an instructor you trust, or train yourself to constantly supply them. Better yet. Do both.