We've got the one with the lift off sections but find that they head back into the 'ready' compost if the weather is particularly cold or damp. The solution in this case seems to be to add more newspaper/fibrous material to the top layer and to put on a new blanket which encourages them back up. I also have a bag of worm treat which seems to balance the womery out if things aren't going to plan - it came with the COW and I've only used about 1/4 of the bag in 2 years.

Not sure if this advice would work in the same way with the other sort, but it might be worth a try. Or check out the Wiggly Wigglers website for more tips ...