Freezer not holding temp & ice on compressor

[DaisyIces]

Hallo
I have no technical knowledge so please bear with me and any advice would be very very welcome.
I have a large freezer that operates like a domestic chest freezer, it has a manual defrost and the compressor and evaporator are outside the freezer box. The compressor - it has the black shell that gets warm think I'm right this is the compressor has a thin silver metal tube wire that goes into it this has a block of ice around it where it goes into the compressor it under the not very good insulation.
The freezer was working very well it is a year old it was off and unused for 5 months, it was switched back on 2 weeks ago and was fine, then it was of again, now it has gone back on it won't reach the set temp ans then cycle it's just continually trying to get the temp down.

The only thing that has changed is an increase in the airflow around the compressor and evaporator so it is now in a cooler environment.

Any ideas what's happened? And how I can solve it.

[Magoo]

possibly not defrosting or defrosting correctly, a block defrost drain.

[DaisyIces]

It's a manual defrost and there is no drain

[Royal241]

Increase defrost time

[1mikeefc1]

Is there an evap fan on it? Is it going round?