Do you use FS9 or FSX or P3D?
FSUIPC has a limit in the number of hotkeys it can allow addons to use. I think this limit has increased at some point, so make sure you have a recent installation of FSUIPC.
Do you have other addons that add extra hotkeys? If yes, you may need to disable some of the other addons' or PF3's hotkeys.
For PF3 you can click on Options #1 -> Hotkey Options, then deselect the keys you don't need, e.g.:
ESC key (if you do not use voice recognition)
Ctrl-Shift-C (if you do not want to toggle closed captions during flight)
Ctrl-Shift-E (if you do not want to declare emergencies)
Ctrl-Shift-\ (if you use VCP mode 2 or 3, and therefore do not need the Roger key)
etc., or disable other addons that need those hotkeys completely.
P.S.: This is the old PFE sub-forum, there is a different sub-forum for PF3 (click on Board index to find it)