Since your symtom is general, I'll answer as such.
An air leak due to doors not sealing can cause general icing. A defrost cycle that is too long can cause icing in areas other than the evap. A defrost cycle that is too short can cause evap icing. Intermittent problem with the evap fan. Overloading the unit with too much product can restrict air flow and cause icing. Door not closing due to spring loaded door closing hinge not working correctly. I'd start with some of these suggestions to see if we can get you in a direction. Your freezer should be able to handle a minus 5 to 10 degrees, generally speaking.
Answers & Comments
Since your symtom is general, I'll answer as such.
An air leak due to doors not sealing can cause general icing. A defrost cycle that is too long can cause icing in areas other than the evap. A defrost cycle that is too short can cause evap icing. Intermittent problem with the evap fan. Overloading the unit with too much product can restrict air flow and cause icing. Door not closing due to spring loaded door closing hinge not working correctly.
I'd start with some of these suggestions to see if we can get you in a direction.
Your freezer should be able to handle a minus 5 to 10 degrees, generally speaking.
1. reduce the Thermostat switch and see.
2. There can be a air leak from the door gasket.