The thermostat may be sticking closed, or partially, which would let the engine get too hot. That would prevent the hot coolant from getting to the radiator as well.
There could also be an air pocket in the system, or you haven't bled it properly.
I'm trying to visualize your car, but I'm not sure..........there should be a bleeder screw somewhere. Is your expansion tank part of the radiator? If so, the screw is next to the cap. Open the bleeder, and fill the system, with engine off, until coolant flows out of the bleeder. Tighten it up, and then run the car at about 1500 rpms with the heater on full temp. That should get the air out of the system.