Refrigerator not getting cold

"If a refrigerator is low on refrigerant that means it has a leak. The sealed system is sealed and should not leak. When there is a leak, air and moisture enter the system. That is not good. It will cause the copper lines to erode and flake. This will cause a restriction in refrigerant flow. A restriction will cause the refrigerant lines to freeze up. When the refrigerant doesn’t flow properly your refrigerator won’t cool properly

You should not see frost on any copper lines. I would have the technician come back out and check the system. If it was me, i would recover the refrigerant, replace the filter dryer, repair all leaks, pull a vacuum to remove all moisture, then recharge the system."