How do I get past Liberty on Ryloth?

I get to the very end, and go to blast open the doors, and the cannon will not hit the doors no matter where I place it, even if it shows that its locked on. This is very frustrating because I've now almost coompleted this mission twice and had this problem. I use a silver cannon even! It will hit slightly to the left of the door and slightly above the door, but no matter what I try, it doesn't work.