blockage or insufficient coolant.  for blockage, try a reverse flush.
for leakage, check inside radiator for fluid level - if there's a leak,
the expansion bottle won't show it because air sucks back through the
leak rather than fluid from the bottle.

In my daughter's 1993, it was the adjustment of the mixer box, at the very
bottom of the center console. I know yours may be different, but see if
these directions apply...
Remove the cover on the passenger (right) side of the center console. You
need to move the carpet out of the way, and the cover is mostly comprised of
a duct, but it comes off easily. IIRC there are one or two screws, and it
sort of pops off. You will see two cables; one goes to the temperature
control knob and the other goes to your heater control valve in the engine
compartment. Turn the heater control valve and watch things move. It will be
obvious what is or is not in adjustment; the adjustments are done by
unclipping the cable housing restraints and reclipping when the cable
housing is in the right position. (It really is easier to see than to
describe.)

If the cable to the heater control valve doesn't want to move, you can try a
spray of silicone lubricant on the top of the valve (near the firewall) and
exercise the valve with your fingers. If you have to replace the valve, be
smarter than I was and pad the edge of the catalytic converter heat shield
before prying the hoses off. I almost needed stitches for that one. If my
wife had been here, she would have insisted on them :-(  I was just happy I
hadn't cut any tendons. Anyway, be smarter than that.

Mike