Atziri's Splendour can't socket new soul core

Have an atziri's splendour with "This item gains bonuses from socketed soul cores as though it was also boots". It currently has 6 Soul core of tacati's socketed but when I try to replace one with Guatelitzi's thesis, nothing happens. Guatelitzi's thesis is labeled as a soul core when you hover over it and when you search it in currency exchange.