On some of the longer levels I've noticed my towers get "stuck" after long periods of firing. Usually these are level 3 cannon or meteor towers. When there are enemies nearby they won't fire or rotate to track the target. Most noticeable on grinder maps.
Here's a screenshot:
The level 3 cannon in the upper right was just frozen in place and wasn't engaging the turtle as it went by.
Selling the turret and building another is the only way to get that slot working again.