This problem recurs in my fridge/freezer and I resolve it by taking out the inner panel of the freezer compartment to expose the freezer unit at the back of the freezer. This unit ices up occasionally and becomes a solid block of ice, the result of which is to severely impair the function of the freezer. Defrost until there is no sign of ice between the freezer pipes and fins etc, replace the cover and switch on. In my case with a Beko freezer, this does the trick and it works like new after defrosting.