why does my white knight tumble dryer only work on high heat setting??

I presume you mean it doesn't heat on low heat setting, if so then one of the tracks of your 2 track element is faulty. On low heat it brings on just 1 element, on high heat it brings on both elements. If you have a track faulty it will not heat at all on low and only heat one track on high instead of 2. If, as your question suggests the machine does not operate at all on low setting it will be another problem.

Kenjo
March 2015