IMO clean with brake cleaner and scrub until no residue remains then apply super glue to the inside of the grip and slide onto the bar, let setup for a few minutes and you're good to go.
If that doesn't work the grip needs to be replaced. Looking at the pic I'd say time for a new set of grips !
If that doesn't work the grip needs to be replaced. Looking at the pic I'd say time for a new set of grips !