1994 Geo Prizm Heater Core problem

I recently installed a new head with all new hoses, thermostat, radiator etc.. Before the heater would not heat at engine idle. Worked ok on the road. Now it doesn't heat at all. I checked the input and output ports on the firewall. I do have hot water going into the heater core and returning. I also tried running water both ways thru the heater core with a garden hose. It runs thru clear and clean. There are no leaks at all. Any idea what might be the problem? Your thoughts will be greatly appreciated.
Thanks