I can’t speak for all of them but the Creality ones that I have have two parts. There’s a sticky (as in peel-and-stick) part that attaches to the aluminium bed (remember: aluminium is paramagnetic, not ferromagnetic - there’s your science lesson for the day). The stick-on is a ferromagnetic sheet. The magnetic build surface then sticks to that. In this photo, they yellow is the peel-off portion, next to the corresponding build plate.
You’ve probably already got a peel-and-stick one on there or you’d be asking why it doesn’t stick at all, not just flat, so on to part two:
If you already have a ferromagnetic surface peeled-and-stuck to your build plate, it may not be compatible with your magnetic build surface. Again, I don’t have any definitive experience with this (ie. I’ve never tried mismatched stick-on and build surfaces), but I have observed that the Creality ones seem to have a magnetic pattern built into them. If you take a Creality surface and place it on a Creality bed, you’ll note that it very much wants to align itself in a particular direction. For example, you can’t just shift it 2mm - it has to be 5mm. It’s like there are magnetic ridges that have to align themselves. If you rotate the sheet slightly, the ridges won’t line up and you’ll end up with bulges.
That’s as far as I can demonstrate because both my magnetic peel-and-stick surface and my build surface come from the same manufacturer.
The point here is that there is evidence for an alignment pattern, so if that’s the case, what if they alignment pattern on your peel-and-stick surface is mis-matched to your build surface (different manufacturer, perhaps)? Then I’d imagine you could end up with regions where the magnets repel each other, even if the build plate is lined-up and centered. Your steel office door and my whiteboard will never have this problem.
I’d suggest a peel-and-stick thin iron sheet to attache the magnetic build surface to. It’ll only have half the magnetism (since only one surface is actually magnetic) and so it will be prone to misalignment but that probably doesn’t matter, but it might also be prone to shifting since the magnetic ridges of a paired stick-on and magnetic base would never allow it and that may be by design. I have to presume Creality et-al went with a two-part magnet on aluminum vs. a thin steel sheet for a reason.