There are so many pills of different shapes and sizes this can happen occasionally.
Suggested solutions for stuck pills:
Cut the problem pill into 2 halves
Change the pill to another of different shape (if possible - ask your gp or pharmacist)
Program 2 or more compartments to present at the same alarm time
Leave non-time sensitive medication out for prompting at the same time as those in the dispenser.
I hope this advice is helpful.
With kind regards
Nicky