Since you have been taking her there since she was little I don't think it is an enviromental problem. She retrieves dummys in the back yard and she retrieves ducks but she won't fetch in the field? Sounds like she does not know what is expected of her. I would suggest that you for the next week or two (or however long it takes) work on obidience and retrieves only in that field. I would let her do whatever she wants for the first 15 minutes, let her blow the nervous energy off then make her sit and stay (my guess is she'll try to break) while you walk around or just stand there. Make her heel (off lead) and walk around the field, if she breaks sit her down and start again. When she starts paying attention to you ( which probably is going to take some work from the sounds of it) then you can work on your retrieves. Frankly I think the dog is just not paying attention to you and untill you accomplish that everything else will just be when the dog feels like it. By the way, if you can stand your dog in a field with 50 other dogs and the dog keeps their eyes on you, waiting for your commands, you'll know you're making progress.