Just to make sure: When the VoiceAttack command doesn't appear to be registering ingame, do the physical keyboard keys the command would press register?
Presumably you're using a joystick; are any buttons being held down (E.G. by physical toggle switches)?
DCS has a feature that allows joystick buttons to function as modifiers (in the same way Ctrl, Alt, Shift, and the Windows keys are modifiers).
If any of the buttons being held down are configured as such a modifier, when VoiceAttack pressed the F key, that would register as the modifier + F, and as such have a different effect (likely no effect at all if you haven't assigned anything to that combination).
If you open the ingame DCS controls configuration menu and execute a command, the corresponding ingame binding should be highlighted (if the key/button combination is assigned to anything); is that working?