Is it because the set closing time is too early for your chickens returning to the coop, or do your chickens not return to the roost at a pretty regular time?
I manually operated mine for the first few days until I learned ,my chicken's habits, then set the timer to open about an hour after daylight, and closing about 15 minutes after they all regularly returned to the coop at night.
Depending upon the predator situation in your area, you may just have to program it to close a while after dark to ensure they have all gone to roost.
Due to the recent time change and shortening days, I just reset mine yesterday to deal with the shrinking daylight hours. I figure I will have to adjust one more time by the end of November, and then I should be good for a while.
Hope this helps.