Thermostat is the first thing to check, since you said the temp
gauge doesn't hit the halfway point.  Easy and inexpensive job.

 Then worry about water flow through the heater core - check that the
heater control valve opens like it should, hoses are clear and not
collapsed internally or kinked at bends, backflush the heater core
with a garden hose to make sure it isn't plugged up.

 Water pump next - when they're worn, the impeller rusts away and
they don't give the flow volume.  Might be enough flow to keep from
overheating, but not enough to push through the heater too.  But
changing water pumps can be complicated.

 Last resort is swapping out the heater core for a new one, because
on most cars and trucks it's a Gold Plated SOB of a complex job.

   --<< Bruce >>--