I asked by nerdy scientist husband about this as I always thought copper were best and I'd never heard of having to take a magnet shopping. Big mistake. Evening of gobbledygook on ferous metals, induction science, phasing, magnetic circuits etc. Made my ears bleed.
There was an answer in there somewhere. I think it was that you have to use iron or steel. Copper, aluminium or glass won't work, unless they are specially made for induction hobs whereby they to make them work.