Cannot give Intimidate ability to Landorus-Therian

I caught Landorus, and I used the Dream Mist via the Dream Guru to give it its Hidden Ability, Sheer Force. After that I changed its forme to Landorus-Therian, and it retained Sheer Force. I can use Dream Mist on it again, but only via the Dream Guru, and it does nothing. Using the Ability Capsule does nothing, and I can't even use the Dream Mist on it via Wake Guru.