Why is my defrost cycle inconsistent?

Fridge-Amana- ARB2517CSR

My defrost cycle is inconsistent. The defrost cycle only works every 4-5 days. The defrost timer seems fine(Check for continuity and the compressor turns on after 30 minutes).

I suspect it could be the thermostat The defrost cycle only works when the freezer/fridge is at its coldest setting when the coils are basically frozen solid(which takes about 3-4 days). Only then, the freezer will defrost itself instead of the usual once every 10 hours.

Anyone had this problem before?