I noticed a problem when you used a bonus...you seem to lose control of the characters at the start of the level and not be able to do anything. I also noticed how to fix it, when you start the level press TAB and click in the yellow square that shows up - you should be able to move after that.
This game would be so much better if there was a little more space in some of the levels... I got stuck so many times I kept having to reset and it became a real nuisance.