Not sure if this came across in my previous post, but just to make sure:
Magnets have to be flush with the surface they're supposed to adhere to. Just glueing them to the plastic on most bases will not work, since you have a gap between the bottom of the magnet and the steel sheet.
You either need some sort of spacer, or my preferred method a modeling putty (greenstuff, milliput). Wet a section of your work surface, make a little ball of putty, put the putty in the base (preferably under a point where the miniature attaches), put the magnet on the putty, and then push the base against the table to get a perfectly flush magnet.
The greenstuff will lose its hold to the plastic, and to the magnet. A drop of superglue works fine to fix this, but it's annoying to have to go back and do that. A drop on the base where you will stick the putty generally works, but if you put glue on the putty/magnet joint you'll make a mess on your table. I usually go back with a thin superglue and brush it onto the magnet joint, but that's not an optimal solution yet.