Out of about 10, I've had two disappear from I suspect lack money. Two just didn't have the hands to handle gliders, although they were both rated in power. I never flew with them in power but heard questionable stories.
One never made it to solo, could not handle or figure out the cross controlled stall. The other failed a check ride for the no spoiler approach. Over 40 patterns with 3 different instructors and he still coud not do it, had created a personal mental block on the maneuver, he had an excuse for every pattern, wind changed, no wind, opposite direction (left vs right patter). He quit, blamed the inept instructors and left.
6 passed with flying colors.