Where's the heat?

A couple years ago, I had a new 3.1 engine put in my 1995 Olds Cutlass Supreme. At the time, I replaced the radiator, the hoses, and the thermostat. Everything but the heater core. Since then, I’ve put in another 190? thermostad and had the heater core flushed. Still, it takes a long time to heat up the car. This morning, I noticed that I have okay heat coming from the upper vents but not from the lower ones. What gives?

P.S. The anti-freeze levels are fine. (Oops, I accidentally clicked on the “inappropriate” flag).