Sounds as though you need to bring the dog on properly.If its not interested in chasing it while your out and you send it off to get it, then that can lead to what your describing. I think your putting to much into it when you take him out , see how he was when he chased the cat , and how you were with him at the time aswell. Then try and duplicate that out in the field. Instead of telling the dog to go chase a rabbit , let it go chase one on its own accord. The dog has got to want to do it and you can help him get that way if you see what i'm saying.