Does it fire (cannon sound, gun muzzle flashes)?

If yes, you have a recoil circuit issue. It could either be an electrical issue (broken wires, loose connections, stuck microswitch) or a mechanical issue (jammed recoil gearbox, gun barrel too tight in barrel sleeve).

If no, you probably have a radio configuration issue. Re-run the MFU configuration, and take particular note not to push the stick all the way up or down for the elevator channel (push it only 2/3rds the way). That's a simple way to "cheat" the MFU into firing the guns.