First of all, congratulations on being somewhere that is cool enough that heat is a concern. Yesterday was in the high 90s with about the same humidity where I am. Whew!

Secondly, there could be several things going on. Start by checking the engine coolant level. The heater core is the highest component in the cooling system and will empty out when everything else appears to be okay. Check the coolant level in the reservoir. It should be at the full mark. After you are sure the coolant level is okay, start a cold engine and monitor the heater hose temperatures as the engine warms up. They should warm up equally and be pretty warm in about 5 to 10 minutes. Once they are warmed up check the vent temperatures on the full heat setting and high blower. If the air is not 120 F or higher, start looking at the temperature door actuators.  Observe the actuator shaft movement to confirm a 90 degree movement arc from full cold to full hot. You will likely need a flashlight and possibly a mirror to observe this movement.