Maybe you don't really want to play it, or have got bored with it? Might sound stupid but I remember me and my mates playing some rock stuff all day and we were bored with it. At one point the drummer just went into this reggae type beat, next thing you know we were all enjoying ourselves and playing a lot better. I saw this tip in Guitar Player mag which says the same thing but far more eloquently:
"Play what you would like to hear, rather than what you would like to play.” —Bill Kirchen