Kayn Unable to transform after picking Assassin initially

So I just finished a game with Kayn and I picked Assassin first because that's what I was forced to pick. I'm waiting and waiting to switch the Darkin form because the other team is tanky brawler champs and I figured I lucked out. For the rest of the game I got no chance to swap forms. Is this a glitch with only Assassin as your first pick or can other people not swap forms from Darkin to Assassin?