If you feel the back wall inside the fridge does it feel cold or the way down or is it iceing up?
Or is it just cold towards the top?
Ok im afraid you have a fault on the sealed system side as its not pumping the gas all the way round the system fully as the ice should be down nearly all the back. As its not thats why its not as cold as it should be.
For this you need to get a fridge engineer in as its not a diy fix. As they need to put there gauges on the back to see what pressure the compressor is pumping at.
But this isnt a cheap fix. If its just a regass then you are looking at about the £140 mark. But if it needs a compressor then you are looking at about £250 so not a cheap fix.
Sorry its not good news but if you need anymore info just get back to me.