The mechanics behind Zexion's card theft
Read the chapters to know what's going on.
Copy pasted from the wiki :
"Zexion switches to using the stolen Soul Eater cards after stealing at least 1 card, and when Riku has 6 or fewer reloadable cards remaining.
Zexion's card and sleight have no effect other than dealing damage if Riku has 6 or fewer reloadable cards remaining. Therefore he cannot ever switch to using Riku's cards if he hasn't stolen any before then.
After Riku has destroyed all floating open Lexicons and retrieved enough of his stolen cards, Zexion switches back to stealing cards."
Chapters:
0:00 Spamming sleights to get as few reloadable cards as needed.
2:38 Starts using his only stolen card
3:23 Can't steal anything anymore