Some weren't magnetic, and some have very weak magnets.
My first replacement had a weak magnet almost nil in fact. Second was a bit stronger.
I got the old one, cleaned it out and scratched it up a bit on the inside for the glue to stick, and filled it with JB Weld (their slow stuff). Then I bought one of those small rare-earth magnets from Radio Shack about the size of a pencil eraser, measured the O.D. of it, and drilled a slightly larger hole in the JB Weld for it to fit. Mixed up some more JB Weld and stuck it in the hole and flush with the plug's end. It will suck a Buick onto it now.
Only reason to do the glue and pre-drilling is the damn little magnet will constantly stick to the wall of the drain plug if you try and glue it in the first time, or creep through the stuff and get buried in the plug someplace deep. Seems from the plugs I've got and dug through, even BMW had issues getting the magnet centered or it got lost in their gluing procedure.