We have had the odd magnet come loose, after trying a few types of adhesives we found Araldite 2014 (RS part no. 372-174 is about as close as we have found to Astroflights original glue.
You need to carefully remove any old glue from both the magnet and motor body, make sure both surfaces are TOTALLY greese and dust free and once the magntet is in place leave well alone for at least 24 hours for the Araldite to cure.
Hope this helps.