Ties mission cannon not firing

[zenchronus]

Either this is a bug or I missed how to fire the cannon. As you can see in the screenshot I have cleared all mire, have the energy mine and the cannons are fully charged. All 4 have been sitting there limply for about 10 minutes as I progress through the level. Clicking on them yields no buttons and they seem to not want to auto fire. Am I doing it wrong or is this a bug?

[Karsten75]

There doesn't seem to be any enemies in range for them to fire at.

[zenchronus]

I thought that might be the case. I never saw them fire and didn't see a range when hovering over them. I assumed they would cover the mission map. I had probably cleared out the enemies by the time I took them. They were's super useful for this mission the way I played it I guess.