I have been working with my 16 month old Lab Collie mix for about 6 weeks now. She was doing really well with multiple marks on land and water I had her going off her name and it was working great. I started to work on whistle and back with her about two weeks ago while starting every session with marks just as a refresher and to get her zoned in, then I would work on whistle and back command for a bit and when she would understand it really well I would finish up with a couple more marks then we would be done. Everything was still gong really well she would go on her name every time. This is where I think I messed up. I started to through her a mark and send her on it, then I would stop her with the whistle half way and send her back after it. things were still fine until this Monday all the sudden she wont go for a mark on her name, she wont even go at all until I give her a little nudge behind the head then she will take off fine. I think she isn't going anymore because she thought me stopping her with the whistle was because she was doing something wrong and now she is confused. So I separated whistle and back from marks again to try and get her to leave on her name again. Am I going about this the right way or where did I go wrong?