I do not know why but at the beginning of every scenario, there is no cards available to edit my characters deck. I can only edit new characters at the beginning of Brigandoom but the items I've gotten out of chests are missing, I can't even edit in basic spells or items at the beginning of any scenario or even at the beginning of adventures...

It is in the rules. You can only Edit deck in the very Beginning of the game. You can not Edit your deck later in the game except if:

- you have too Many cards: discard extra cards

- you have not enough cards: add cards that Are two level below you from the box

 

So there is only one chance to Edit your deck fully in The game. After that you can only tweak it a Little based on what you find and what you lose during the game.