Izzy is the same way with the wet grass but she is getting so much better at it. She is tiny and so low to the ground that we also found out that if the grass is too high she won't go on it either.
Since it snows here a lot too she won't go (or can't go) in the deep snow so we have to shovel a clear path for her to go from the deck to the drive. That she has no problem in going but wet grass..hmmm..that's another story....but she will do her business on the driveway..go figure.