Can't add some cards to decks - no specific criteria

Yesterday was trying to submit a new deck and couldn't add Fei  or TigerVespamon Ace.

Today I was trying to make changes to my Eater deck and couldn't add Erika Mishima .

I don't know what the criteria is, there are just some cards I can't seem to add to decks.

This is fixed now! We had a bug in our deckbuilder.