Some keyboards have certain key combinations that cannot be pressed together. This is because the manufacturer is looking for a way to save on costs, and they think that certain key combos would very rarely be used (aside from in FoF obviously). The only way around this (short of buying a new keyboard) is to remap your keys. Usually using 1-5 rather than the f1-f5 keys works. That, or try switching which key is used to strum. Some people have found that assigning strum to f12 solves the problem.