I have one of those basic worm bins that councils often promote (open
bottom, narrower at the top, hatch on the side at the bottom for
shovelling out), and I find it quite difficult to get all the good
stuff out of the bottom without unfinished stuff collapsing down.

I use a spade to scoop the finished stuff out, but end up with a
cavity with a sort of arch over it, and it's very difficult to scrape
out the finished stuff from the sides and back. Then a few days
later, it drops down but the stuff accessible from the hatch is
largely undigested and I end up shovelling it back in the top to get
at a bit right on the bottom.

Any tips? Maybe a spade isn't the right approach?