Hello Daniel to see if we clarify or I am clear. Consider that a plate may have frost but with a very poor cold, the pressure of -2psi would be good for the r134, but put it to work and check if it frosts the entire plate or if you have a cold suction pipe it would be good for gas. On the other hand you have to look for if there is no open door or broken gaskets ((always talking about cyclic refrigerator or frost)) not a NO FROST, since you do not clarify what it is. Another thing is or can be an uncalibrated automatic that cuts in a short time or is in a minimum cold position. It could also have a condenser near the wall or heat sources nearby. Do the following connect the compressor directly or bypass the automatic and let it run 2 hours and add a thermometer to the bottom well attached or attached to the plate and take the temperature and see how much it arrives and comment.