I have the same problem with them and that was on a fresh terrain with no cache involved.
There is one airfield per country on my terrain that has a spawn point on the other side of a river. Enemy GVs have to cross one of four bridges to reach the airfield. The bridges belong to the owners of the airfield and so cannot be bombed to prevent the enemy GVs crossing. There's not much point for the attacking side to bomb them of course.If they were bombable from the defending field I figured you would get milk runners constantly bombing them for points as they are only a mile or so away.There appear to be so many problems with rivers and bridges though, I'm coming to the conclusion that the only way this terrain will ever get accepted is if I ditch them altogether.