When there are more than two layers the magnet just falls off, or slides down the fabric during the day. Am I using them wrong, or are my magnets too weak?
Two things. One, thin single magnets cannot hold thick or textured fabric, you need the stronger multi-layer ones. Two, magnets slipping down usually means the fabric is too silky, so the magnet has nothing to bite. On heavy fabric I actually get better hold from an instant hijab that has the magnets built in at the right spot, no fighting it. The magnets vs pins guide explains the strength thing well.
Place the magnet a little lower, over the shoulder seam area rather than right at the jaw. More layers of you underneath = more grip and it stops sliding.
For chunky winter fabric I gave up on magnets and use one good no-snag pin at the drape point. Honestly more reliable once the fabric gets thick.
Full how-to that sorted mine out: the hijab magnets guide. Turns out I was using ones half the strength I needed.
Update: stronger magnets plus placing them lower fixed it completely. Thank you all, I nearly gave up on magnets entirely.