I would assume that you're fishing those crab patterns in the upper water column with a floating line? Some of us use these same flies with sinking lines. I think that's the time, in this situation, when fish tend to get hooked deeper into their mouth cavity. They have their mouth to the ground, verus them taking a floating fly and heading toward the bottom.
Once had a fisher friend with a drinking problem. One time a floating empty of a Henkie bottle came floating by. As him why he was thrashing the place? He told me that was his strike indicator?