It was funny when the environment changed. I agree with the other review: sometimes you feel that you didn't click even when you did. I don't think it's a bug, but perhaps it's simply that things are going fast and you miss the sheep a bit. Anyway, check that the collisions are good enough; given the pace of the game, this is important.
Giving some more variation would be nice. Giant sheeps that require multiple touches or something. Also, marking the already touched sheep would be useful.
Try to make a mobile version!