Sometimes an interesting face will come after a few hours of drawing more familiar ones.  My equivalent to getting caught in a drawing rut means that I either 1) don’t draw at all for long stretches of time, or 2) draw the same things over and over again—the same noses, eyes, lips, and expressions.  Maybe this kind of familiarity work is not so bad; it’s comforting and, I think, is better than not exercising the art chops at all.  It can also, after hours, lead to the kind of familiarity fatigue (boredom?) that produces something new.