I have three 'Dalek' compost bins at the end of the garden. They are
standing on the ground, side-by-side, in a row.

They all get fed with the same sort of stuff (kitchen vegetable waste,
dead plants, small garden material etc).

Whenever I go to put something in them, more often than not when I lift
the lid of the middle one there's a mass of worms having what looks like
a serious sex orgy (and there are also quite a lot of them on the
underside of the lid). It is very satisfying to see them all having such
a good time. However, in the other two bins, there are almost never any
worms to be seen.

If I transfer some worms from the middle bin to the other two (usually
by swapping the lids), next time I visit the bins the situation is
exactly as it was before (ie no visible worm activity in the two outer
bins). Despite this, the contents of all three bins seems to be
composting away at about the same rate.

So the question is, "What's going on"?
--
Ian