So, I put Marv into a gunnery console but he won't shoot at the enemies. And suggestions?

When I man the same console, I shoot alright. The training mission works fine. But now it doesn't work. I tried this across multiple ships, no luck. The worst is that Boulder 380k one, where Marv is meant to man the heavy guns while I have the machineguns - and Marv won't shoot. Grr.