Could also possibly be over-charged if the
lines are freezing up.
This passage is from Frigoboat:
"Too much refrigerant in the system will result in liquid still evaporating back to a gas past the end of the evaporator and inside the tubing going back to the compressor. This means that there is still some of the refrigeration process going on inside the tubing and there will be a
build-up of frost or ice on the exposed section. If additional insulation has been added, it may be concealing this symptom and should be removed. The evaporator may appear and sound normal, but will be at a higher temperature than desired, resulting in longer than expected run times."
These links could help:
RParts: Refrigeration Parts Solution
Design Your System