Stars have to be assigned to powers between levels. You should have allocated the force stars BEFORE beginning the level (once you complete the previous level, you should get a cutscene, then a screen with Secrets found etc, then the Force Power screen. This is where you should allocate your stars (It is impossible to do this mid-game, as you have obviously found out :-) )
Hope that's of some use to you.
- My Personal Starcruiser.