Problem is, the needle is an inch long so it injects an inch deep. Magnetic force works according as a square law so the attraction force at 1 inch is about a hundred times weaker than the force at 1/20 inch deep.

And if you are suggesting that ferromagnetic particles managed to creep back up to hide just under the skin, I'd like you to calculate how many grams it would take to hold a neodimium magnet there. Just as a guess, I reckon it would take at least a pin-head of iron (or cobalt) but probably more, even accounting for skin friction to prevent its slipping. That's a lot of molecules.