Emperor of Bones

Emperor of Bones isn't working correct. It only worked the first time it entered the battlefield. I cast one and it gave me a card with 7 mana. Immediately following 2 more were cast while I had 7 or 8 cards in my graveyard and none of those folling Emperors brought back any cards from my graveyard and then in the following turns, I cast other skeleton cards while the Emperor was still on the board, and they didn't bring back cards from my graveyard either, and yes I had room in my hand.