I've put 10 drones in the depot but one 1 does any work. Can't figure out what is going wrong.

Have they already filled up the depot? The depot tries to balance the number of drones out to only have a few thousand ore

Ah yes. I've just started Ballast's Bolognese Modpack with a Trains map which has copper rocks or whatever they're called near the starting area. Those rocks have a lot of copper in them but the mining drones don't or don't seem to take a count of that; probably just the area size.
But I see the problem and many thanks for the prompt reply Septimusii