I have no info on how to rejuvinate the sheen of foil. That's why people usually case their foils and use proxies instead. I've tried myself with foil cards from other games and it all boiled down to "I should've taken better care of my stuff."
Are your foils losing their gleam even when double sleeved?
There have been quite a few reports at my locals that WIXOSS foils tend to 'rust' away, but I didn't think it was due to the way the cards were made. I used to think it was because people didn't take care of them, but this is new information.
You can't rejuvenate the sheen of foils. It's difficult as it is to duplicate the printing process of cards, let alone trying to recover the foils of cards which were already printed.