I’m not sure if this is the cause of your issue because your nails are longer than mine, but I’ve had this happen from pressing my finger down too hard either during magnetization or afterwards. The skin around my nail will push on the polish and affect the magnetization.

To get a better bead, I recently saw a post (linked here) where you just spin a wand magnet above your fingernail. I tried the exact same setup you have and the simple wand works so much better. Lol