Removing the LU to back-flush thru the stat openings will allow a much greater flow volume than with the LU in place.
Next time you have the LU down give it a try just to be sure.
Also, when the lower is down rig a garden hose directly to the water transfer pipe, if it fits well you may be able to look up to the upper end of the pipe's fit in the grommet in the adaptor plate to see if it is leaking or well sealed as it should be.
