Originally Posted by INSIDIOUS

Does the upper return vent seem to be sucking air as strongly as the lower one? You can temporarily block either with a newspaper or something to see if the other is sucking.

The other obvious thing is to be sure the stat for upstairs is calling for cool so that it opens those dampers. Switched to 'cool' and turned down. Does cranking it either way have an effect?

Temp aid to equalize is block half the lower return forcing more of the conditioned air to pass the upper level.