1990 Toyota Camry coolant resivior

Coolant is entering the coolant resivior but it willl not syphin back to the radiator. It goes into the resivior and then oerflows from there. This vehicle has a new after market radiator and new cap, along with new hoses installed about 45 days ago. The termostate and inlet-outlet gaskets an hoses were replaced about (6) months ago. There are no leaks from any of the cooling systym. The headgasket was replaced (6) months ago. There are no noticeable leaks from there or from the water pump. However this problem was occurring prior to the radiator and systym replacment. What is causing this problem and how do I fix it? Please respond a.S.A.P. Thanx big -t