Check the door seal. This is usually a concertena like gasket with a
magnetic face to pull the seal against the body of the fridge. Firstly run a
damp cloth all the way around the seal, especially between the folds of the
soft seal, to ensure no gunk is stopping the seal and the seal can expand.
Usually the force needed to open the door is a good guide to how good the
door seal is.
If the insualtion is sound and the gas hasn't leaked out then have a look
inside the fridge to identify the location of the cooling element. In my
stand alone fridge it is under the ice box at the top. If this is covered in
ice then this can significantly reduce the efficiciency of the fridge as it
can prevent the circualtion of air around the coils.
Also if the temperture sensor often a small bead or tube near the cooling
coil (to control the cooling cycle) is encased in ice then it wiull also
give false readings.
I would highly recommend a fridge thermometer as a matter of course (you
probably already have one) to monitor you food storage.
Hope this helps