Remove it from the mirror, and clean it to perfection, and also the glass, preferably with Meths, discard any fitting kits that you have bought... once the seal is broken, air contaminates the primer/adhesive, and becomes US. Buy a Loctite kit from Halfords, to ensure you're not buying old stock. Follow fitting instructions exactly! but only bond the foot plate to the glass, using the thinest and smallest amount... do this with your car facing the sun, preferably in the morning... natural UV light cures the bonding agent, leave 24hrs before you try to re attach the mirror to the foot plate..
HTH
Thom