A loss of refrigerant or a poor compressor can both lead to the last shelf
on the line not getting as cold as the others.
The only way to check would be to check the suction line pressure. Not a job
for the amateur sorry to say. Check the joints around the compressor for
oily deposits which would indicate a refrigerant leak.
If the fridge use R12 gas ( it should say on the comp or inside the fridge
on a label) then it is not usually a viable repair.
Hope this helps.
Eric.