In Co-Op, there is always a chance for one captured objective where the team has to defend. Basically, I believe the defense time is too long, and there are long intervals that nothing happens. Sure the enemy team calls in fire support, but that can be easily dodged by just staying indoors like staying inside from the rain.
So to solve this problem is to either lower the defense time or increase enemy spawn to relentlessly attack for the current defense time. Either suggestion would keep the team on their toes and keep the game moving in a fast pace.