Keezer issues

Hey y'all, I bought an old chest freezer off of craigslist. It is approximately 20 years old (per seller) and rigged it up with a Johnson A419 temperature controller. I purchased this last fall and it has been working great until this past month. The temperature keeps reading in the mid to high 50's even though the controller is set to 36*. Also, there has been an increasing accumulation of ice building only on the upper rim on the hinge side of the inside of the freezer. I am wondering...
1. if anyone can diagnose what's wrong based on this description
2. if the temp control is accurate (contents inside feel approximately the stated temp on controller) why is ice building and not melting?
3. why is ice only building in one spot?

Thanks for the help

I expect the freezer is low on "freon" thus the very first section of the evaporator is the only part contributing any cooling. This is likely a fatal condition as the cost to have a tech attach fittings and recharge the freezer is going to cost a couple/few hundred bucks...

Thank you for the help, I was afraid it was a terminal condition, guess I will have to get a bigger better one to hold more kegs. Now comes the fun part of talking SWMBO into it