Liebherr not defrosting

CNes 3666 Liebherr

Fridge part has stopped auto defrosting. Large build up of ice over time. Defrosted manually today and already a layer of frost building up from the moisture.

I recall the defrost feature used to be a quite loud “cracking noise” when active but no longer hear this.

The drainage pipe is clear of gunk.

Occasionally find water pooling under the veg drawers at the bottom.

Any suggestions on the cause and what part to be replaced?

Thanks