rear a/c '96 suburban

the a/c blows cold from the front vents ,but the rear air isn't cold.i have replaced the expansion valve for the rear unit and still no help.the evaporator coil is only getting cool on the bottom 1/4 of the coils.i have checked to make sure the vent door is closing properly for the rear unit.

my question is ,could the rear evap. coils be clogged in some way or is my compressor bad and unable to handle the load of both units?


thanks in advance for any help offered.