More than likely we have an issue with the blend door actuators, or the heater core is blocked. I would think the heater core is not the issue just because if you can get the temperature hot enough then that points to a blend door.
I am attaching the info below but if it were me, I would access and remove the blend door actuators and move the doors manually to see if the temperature can be achieved. If it can then you have an actuator issue.
One thing to check before any of this is to make sure the coolant is full. If not, then that can cause heating issues.
I suspect we are heading to a blend door issue so let's start with this and go from there.
Images (Click to enlarge)
Wednesday, December 1st, 2021 AT 6:45 PM