About 2 months ago the freezer stopped cooling, so I opened up the back panel in the freezer compartment, saw frost on the evaporator coils. I replaced the defrost thermostat with a new one. The freezer started working properly again, and did so for a few weeks.

The other day, I noticed that freezer temps have risen again, up to 15-20 degree. I haven't removed the back panel yet, but I was able to see through the vents at the bottom that there is frost on the coils again.

I'm not sure what's happening. Is it possible that I got a dud replacement? Or is there some other problem that is knocking out the defrost thermostat? What do you recommend for troubleshooting this problem?

Thanks so much!