whirlpool S20 FSS10-A/G problem

Help please.
Our American style whirlpool fridge S20 FSS10-A/G is misbehaving.
It’s not as cold as it should be. Fans are running. Freezer section is cold but not as cold as it should be. Fridge section isn’t as cold as it should be.
The little common plastic shut off grille in the top left of the fridge was upgraded a couple of years ago to a metal grille that is still intact.
Freezer section is blowing cold air but it seems to be blowing damp air as ice crystals are forming.
This fridge has had many new parts over the last few years.
I’ve got a hunch that the compressor isn’t kicking in. Thermostat, main board, Upper control panel have been replaced a couple of years ago.
It seems the fridge is trying but not working properly.
I’d rather try to fix it that throw it away for the sake of a few quids worth of spare parts.
Please advise on what to investigate and test.
The back lower grille is off and I’ve had a look and hoovered the dust out. Not much to see under there.
Many thanks.