Temperature problems ?

I have an Ocean upright freezer. I noticed that the temperature warning light had come on and assumed the door had been wedged open. I put the freezer to fast freeze and this apparently solved the problem initially. I say apparently because I cannot remember if I put the temperature control back to normal running or not. Anyway in the next day or so the same problem occurred and I set the freezer to fast freeze again. This has not solved the problem. I have placed a thermometer in the top freezer compartment and the temperature is hovering between "freezer" and "fridge". To maintain this temperature the fast freeze operation is going full time. Can you advise please