I think Kakashi didn't use the MS for a long period of time after he unlocked it because he didn't know he had it. He feinted right after he unlocked it. I figure he reactivated it sometime down the road when he visited Obito's grave and felt the depression again. The depression was from him killing Rin and letting Obito die. After feeling these emotions his MS came back. Which would make sense because if I can remember correctly he visited Obito's grave before the fight with Deidara. I'm not sure if it was RIGHT before or a bit before but I know it wasn't too long before he fought Deidara, which is why he didn't know how to skillfully aim it at Deidara.