The heat pump works in heating and cooling mode but it simply will not go into defrost mode.
Why won’t the heat pump go into defrost mode? I find this question quite often in the winter months after the tech has changed multiple defrost boards. This is a very simple problem to check and troubleshoot. Does the unit have 24VAC? Do not say yes because the unit will operate. Yes, the heat pump will work when the “R” or 24VAC is missing. The “R” powers the defrost circuit and the unit will run without it. Much like a straight cool unit, all that is needed to run is “Y” and “C”. Start by checking 24VAC at the defrost board on “C” and “R”.

I highly recommend using needle point leads so as not to damage the wiring harness. Start by checking 24VAC at the defrost board on the plug. 0VAC will be read when this problem occurs. I recommend starting here and working backwards towards the 24VAC source, for example, the transformer in the indoor unit. Working backwards will lead you to the source. This could include a broken wire under a wire nut or a broken thermostat wire.
So, to recap, 0VAC between “C” and “R” = 0 defrost. The unit will run in both heating and cooling modes.