John T wrote:

Never seen a composter with shelves, and wonder if these are causing a
problem?

I've seen them advertised in gardening bumpf. You're supposed to do as
Zaf says - put stuff in the top, then take out the top shelf, let it
fall into the second chamber and put the top shelf back. Then fill the
top again, and when it's full, let the middle fall to the bottom and the
top to the middle. Then fill the top again. Finally, take your
finished compost out at the bottom.

The idea is that the tumbling from section to section is enough to mix
and aerate the compost. Sounds like a gimmick to me!

Peter