>>6721Looked it up and it seems you aren't the only one who has trouble evolving eevee
Both r/pokemon and serebii forums also have noted the issue. The problem is that it isn't consistent.
Some are able to evolve eevee into Sylveon just fine with the National Dex, while others can't even WITH the National Dex. Espeon and Umbreon evolutions are also affected.
Leafeon and Glaceon evolutions still work even before getting the National Dex for some reason.
The common thread here is that the ones with the problem transferred an Eevee from X/Y at the start of the game and used it normally. Much like yourself.
Nobody has any idea if this is intentional or not, but yeah it's pissing some folks off. Nintendo Customer Support's brushing them off because they assume it's game help as opposed to a bug.
Most of them gave up and just transferred their mons over to X/Y via Box (which doesn't reset happiness/friendship apparently), evolved them back there and then re-transferred them back into ORAS.