Depends what metal the dropout is made of. Most forged road drop outs are not brittle and can be bent and easily machined. Perhaps yours closed up a bit by sustaining a blow (i.e. was dropped on a hard surface). You might be able to carefully pry the slot or tap it open (with a drift) or even file it a bit wider, if you need just 1-2mm more space. Just be careful not to distort the rear stays. If it was dropped you might want to check the allignment of the stays.