nobby, you know what I think? it really smells like the "why can't I press some combinations on my keyboard?" problem...
let's say it's not, so he doesn't get a warning about this, but just for fun, let's see what's written in the FAQ:
Q: Why can I only use certain key combinations? e.g. Why won't F3 + F4 + Strum (Enter) work together?
A: Keyboards always have certain key combinations that cannot be pressed together. This saves on manufacturing costs as it is very rare that those combos would need to be used (aside from in FoF obviously).
Only way around this is to remap your keys. Usually using 1-5 rather than the F keys works. That, or try switching which key is used to strum.