A problem I had a few months back that went away has risen again.
Lower level heat pump blower will only run at a very low speed, so the unit can't keep the lower level cool.
Maybe just the motor going bad (I have not checked the A coil as yet, but I don't think it's freezing up)?
This lower level unit is a 4 ton. I had the upper level 3 ton system completely replaced last summer, and kept the blower from that air handler.
Think that blower motor could be swapped into the lower level air handler?
The old upper level air handler was the same brand name, same age, as the lower level unit so the blower motor should be a fit.
The blower for the 3ton is most likely 1/2hp and the 4ton is probably 3/4hp. If the motor is wired for more than one speed see which winding has power to it. The most common wire colors are black for high speed, blue for medium and red for low. Common is usually yellow or white. You should also check the run capacitor, it maybe shot.