Hi

Obviously its easier to give advise if on site as hand eye can usually find fault. I suspect you have a poor pumping compressor. It will reach temp with door closed but as soon as the customer starts opening and closing door it begins to struggle.Gas charge: you wish to achieve a temp of - 21 so you should be evaporating at around 9 psi on R22. If there is to much gas in the system the side of your compressor will be icing.

Regards

bernard