From your comments about the frosted suction line, the test pressures that it says on the nameplate, and the fact that you seem to be more concerned with subcooling than superheat (on what I'm assuming is a unit that has a receiver), I can tell that you don't have much experience with refrigeration.
Does this thing have a receiver? If so, 18°F subcooling seems pretty high. Could this thing be overcharged, or are you sure that it still has the factory charge amount? On receiver systems, subcooling should NOT raise while you are adding refrigerant. If it does, that means that it's way overcharged and that the receiver is completely full. If this system doesn't have a receiver, then disregard what I just said, although 18° of subcooling would still be high even for a non-receiver system.
A plugged up filter drier might also be able to increase subcooling. Try taking a temperature measurement before and after the filter drier to see if it's the same on both sides.
Your head pressure also seems high for the 70° air that you say is around it. Are you sure it's 70° up there? Usually these things get installed in a confined space with poor airflow which causes the condensers to recycle the air over and over again, which causes the temperature to become much higher than that.
If those things actually check out, my next guess would be air infiltration. Is this thing sitting on the box correctly, with no big gaps where non-conditioned air could be getting into the box or the evaporator section? Is the door gasket in good shape and is the door closing correctly?
One last thing to check is the heated relief port thingy that's usually installed above the door. Make sure it's heater is working and that it's not a giant ball of ice inside of it. These things sometimes go bad and that basically creates a big hole where unconditioned air and humidity can infiltrate the box 24/7.
Keep at it, and get back to us with more questions and let us know if you figure it out.