Can anyone offer advice on my ICY problem -

I'm trying to solve an ice build up in a walk in freezer room, the ice builds up on the ceiling and in the corners of the room around the evaporator.

A lot of ice is also building up around the implosion port which is a three flap type and has an operational heater.
The room has been shut down twice over a short period to let the ice melt, and the room be resealed inside and out.

All panels are in good condition and the defrost is operation correctly with drip times and fan delays after defrost.

The room is 3.2 wide x 7 deep with a single fan evap installed at the end of the room blowing towards the door.

The room has curtains for when the door is open, has operating low voltage floor heating and is installed in a shed with no exposure to the sun ????freezer 2.JPGfreezer 1.JPGfreezer 3.JPG