[Mobhack]

There must be a target set for the firing unit to shoot at. Did you set one with the T key?. Otherwise, nothing will happen.

Same if the selected target is out of range of the selected weapon.

The chosen weapon must have 1 or more available shots - all the ammo in the world is of no use if the weapon is showing 0 shots left for this turn.

Also, if your unit is in retreat status or worse, then you will need to rally it back to at least pinned, or nothing will happen.