I restarted my laptop and checked the keyboard yes, but it doesn't seem to help plus CS5 is answering to the numbers when switching opacity for the brushes so it's really curious that it does not work once you try it with the layers... :/
Hopefully it doesn't prevent me from working, but I hope I'll manage to get it fixed!
Can’t say I’ve had that specific problem myself, but you might want to try this: save your shortcuts out as a custom set, then restore the defaults and see if that gets it working again. Then re-load your custom shortcuts and see if the problem is still there. Photoshop can sometimes be glitchy, and doing a reset is a bit like smacking the side of your grandparents’ old Electrohome TV to get it to work.
It's a very curious problem indeed, I'll see if I get other answers to this thread (or maybe one of my teacher will be able to sort it out for me next week)... otherwise I'll probably try reinstalling the app when I have some spare time. But it's a bit annoying >:I