Sandogasa sometimes can't discard a card even though i already hit a character a few times.
Usually, cards that can't be discarded are cards that created by other cards, like from My name is legion.

And if you used Sandogasa in Kotoba boss, it will never discard Kusanagi if there is another item attached to him. And if i remember i also can't discard Chimera on Noz boss if there is another card attached to him.