I think youll find that this is a common problem, but one that is easily overcome.
When the hopper isnt chock full of beans, there isnt enough weight inside the hopper to force the beans into the throat of the grinder. This causes the beans that are in the throat of the grinder to bounce around or popcorn inside the hopper. The easiest solution is to either:
1- fill the hopper with beans to keep a decent amount of weight on the beans in the throat of the grinder. (This has adverse freshness implications though), or
2- put enough beans into the hopper for the shot youre pulling, and put a tamper on top of the beans to prevent the bouncing or popcorning effect.
If the beans do get stuck, you can try and jiggle them with a chop stick or other slim item (ONLY WHEN THE GRINDER HAS STOPPED) "Safety warning" ;)
Hope this helps,