1993 Chevy Corsica heating problems

Heater problem
1993 Chevy Corsica V6 Front Wheel Drive Automatic 110000 miles

Heat in the vehicle only seems to work when on max a/c, or when the car is warm it will start to work on vent or b/l. But otherwise its will just blow clod out of all the settings except max a/c. We replaced the heater core and water pump.

Any ideas?

Thanks

J.