I had this too. I found reloading a save worked for me at least. I also didn't use the mounted gun the time it worked for me, I just fought on foot.
Well, third time is the charm. I reloaded, and instead of going to the left gun first, I went to the one that is "broken" - on the right first and then did the wave of attacks from there. Manning the gun, I basically just shot the hell out off every drop before they got anywhere. Not sure what Kaidan was bothering to do in all that time.

I'm going to do the Priority Mission with the Quarian next I think - I've been putting it off, hoping my paragon score is high enough. Even without reading any spoilers about it, I could have predicted what kind of decision is going to be made there, so I am expecting a high score in paragon or renegade will allow me to push the decision *I* know I want to do.
Does anyone have a good list of side missions - I have only found sporadic info, and my Journal is so cluttered with little crap that I know is just scanning a planet somewhere, I would really like to clean it up before going too much further.