from the news on game : "only the lower rarity cards grant the bonus to the higher rarity cards".
In other words, if you don't have whole mythical team, your attack power is lower than before... Yes, it sucks, yes, you probly going to have a lot of lost during the next battle in tower :/...
EDit: and now i have the same bugs as you... Well, less and less power... Do the dev want to see us leaving the game ^^?