The only clue I could offer is water. Do you water your compost with water
from the tap and it is detrimental? I water mine with water from the butts.
My daughter who has a massive compost area, (A JCB Shovel can get in)
doesn't do anything with hers except turn it and let rain water do the work
and hers is built in the form of a concrete bay. Concrete block sides and a
concrete base. And yes, millions of little pink works :-((

Thanks for all replies. To answer some of the points:

1. Rainwater used to water the compost
2. The bins are open-bottomed on an earth base.
3. Moisture content seems OK.
4. Not cooked by direct sunlight.

I've used the same system for over 40 years and it's only since moving
to my present address that the worms have deserted. Obviously something
is not to their liking :O( I'll persevere.

Jim