GSS25QFMBB Defrost Issues

Hi,
My problem is the rear wall of the freezer section has recently started freezing up. There is about 5 mm ice always on the rear wall, part number 214 below.

Freezer section replacement parts for General Electric GSS25QFMBBB Refrigerator | AppliancePartsPros.com

The issue is contents are not solidifying as good as they used to in the freezer

My question is what parts do I need to replace to get it working correctly?

Any ideas are much appreciated.