I know you are a terrific provider. I KNOW this.
BUT....I don't think it matters how nice the parents are or how much they might "say" they are trying to help. The fact of the matter is that we have to spend 9 hours a day with the CHILD and if that child is negative to the environment then we have to move on.
I have had parents who were really nice people but their child was just not a happy person. That's just who that child WAS and no amount of trying was gonna change that child or make our daycare days happier.
At some point you just have to say "enough is enough" and move on with no guilt.