Can be caused by a number of things. Usually the door is getting tight when open. Could be the sill out of alignment with the top hangars, You should be able to wiggle the door a little side to side when open. Could be the door gibbs get tight when open. Could also be caused by to much spirator on the hoistway door. Also the operator belts can be to tight. Could be a buildup of gunk on the top door hangar track right where the rollers stop. I have these and other things cause this problem. you need to look it over and determine what is causing the door to want to roll off of the limit, or just barely reach the limit. Good Luck.