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 <insert goobledygook> to make them work.