Bosch KGU31125GB Fridge working Freezer defrosting?

Bosch Fridge Freezer year 7 years old. Top refrigerator seems to be working to correct temperature. The freezer drawers are not correct and food gradually defrosting. Have thermometer in bottom drawer but not registering nearly low enough. This happened last year and gradually improved and by time Bosch engineer came and tested it he said temperatures ok and nothing wrong. This is third time of happening. Can you help please?