i was kinda out of line with my earlier reply, sorry to otto i was just thinking out loud. im just a tech that just got into the trade a few yrs back and i am totally interested in this call its a good one to see.
call me a newbie i can accept that (-:
what i learned from watching my senior tech was to eliminate things to the system. i dont like it that the head is only at 120 psi (whats the actual temp on the suction line with your gauges) which is odd that number should be a lot higher for sure. any restriction anywhere in the line would cause frost build up check for that too (pressure drop causing frosting)
before you do anything else make sure the amps on the common side of the compressor is where it should be compared to the rated amps on the name plate to ensure its not low on charge. if this compressor feeds 2 evaps then make sure both systems are calling get superheat reading. ( is it possibe for the compressor not pumping correctly> valves leaking over to the discharge not allowing head to rise?)
if its the head master control that is the problem you will know right away by having the clamp on thermometers and seeing the temp difference from inlet and outlet(s) but it doesnt seem to be the problem --do process of elimination is key-- hope i was informative and not being a pest (-: i really want to see the outcome on this one thanks all for having me think out loud
good luck Otto