Remember that cover works both ways.
If he can’t hit you through it, you can’t hit him either; and while cover can offer great advantages to you, if you don’t maintain Observation (OODA again..”hint hint”) of your opponent, he can maneuver against you at will. This is why military tactics involve suppressing enemy positions with fire while assault elements move into position.
If you duck down behind cover for too long you cant see what your opponent is doing, which is a bad thing. You must strive to use your cover for protection while still being able to observe and engage your opponent.