Have it tested perhaps to make sure. I had the same issue when my radiator went out. None of the coolant was coming out of the overflow tank to fill the radiator. The newer radiator cap I had on there wasn't working correctly. Does yours have that little lock lever on it?

 

No it doesn't have a lock lever do I want one ?

 

Mine that didn't work correctly had one, so I'd say no, lol. If you are positive the cap is good, you might remove the line and make sure it isn't clogged. There are only three parts to that system: the overflow tank, the line and the radiator cap.