We're working on teaching our 5 y/o ACD to retrieve,and I've stalled out early on. Lucy, our ACD, doesn't like to take things in her mouth unless it's a) food, b)alive and squirming, or c)food that's alive and squirming. Thus, she has absolutely no interest in balls, frisbees, or anything other toy that moves if it's not a real live bird, squirrel, rabbit, vole, etc. She responds very well to clicker training, and we've worked with her to the point that she knows touching a ball or frisbee with her nose is good. But I absolutely cannot get her to voluntarily pick up a toy, even for food. We've tried the following:

1) wipe scented or tasty things on the ball (she licks it off without picking it up)
2) put treats inside a ball and show her that the only way to get the treats is to bring the ball back (she shreds the ball to get the treats, and abandons the toy if we correct the shredding)
3) forced "take it" by putting the ball gently in her mouth and telling her "stay" (obviously, this only increases the dislike of the ball being forced into her mouth)

Any suggestions on how to continue shaping these behaviors to entice her to pick up the ball? We really want to teach her fetch to start moving foward in rally training and service dog training.

So, does she dislike the sight of a ball now?

I wonder if you started over with a different object that looks nothing like a ball (like a leather glove, or knotted hand towel, etc) and trained her by shaping the behavior slowly, if she would respond better?

Once you have done that and she knows the command to pick up an object, you could start varying the items to pick up, and eventually introduce the ball again.

What I have been doing to teach my soon to be service dog to "take" is this:

We do a bunch of training first so he is tired and panting and wants to lie or sit down.
Then I take an object and while his mouth is open from panting slip it in pull it RIGHT out and say "yes" and treat him.
Really quick. In and then right out of the mouth.
We did this for a few days. He didn't like it at first, but I kept it short and sweet, 4 tries, train something else 4 more tries etc for the first few days.
lol today my old service dog was near us and wanted to "play" too. so I did "take" with her, and he was watching.....hmmmm....so I alternated between them. He "got" it in about 4 tries. All from watching her.
Up to this point he had been doing ok, making progress, I was happy, but wow. Now he really gets it!
Is there a dogie friend he likes that can do what you are trying to teach?
They do learn from watching each other.

Will she show any interest in a stuffed toy like a bird or squirrel?

Stuffed toys are a big hit around here and if it is stuffed, looks like a squirrell and has a squeeker, we've died and gone to heaven.

A million years ago with my first Border Collie I moved away to college and away from livestock and had to find an outlet for her....so I thought I would teach her to fetch tennis balls. She had the outrun but would circle the ball like a flock of sheep and then come back with nothing but a hopeful look on her face. So I spent the next month or so running after the ball with her, putting it in her mouth....with tons of praise, holding it there and running back to the place we started. Again, tons of praise as I released her mouth and she dropped the ball and looked at me like I was insane. We obviously didn't throw the ball all that far at first....as I already looked like an idiot at a short distance. Of course, being a BC she obsessed over tennis balls to her dying day. I learned to do my college work with one hand soggy with dog drool.
With an ACD.....not sure if once they latch on to an idea they are as obsessive (well, nothing is as OCD as a BC) but if I had it to do again (and have with other dogs since) I would teach them to retrieve a multitude of objects before they obsess on JUST green flannel.
I did teach my Bullmastiffs to retrieve the same way. They did learn it...but only did it because I asked and were SO above doing it for fun. "You threw it away. Didn't you want it either?"

Depending on the jealousy factor you may have good luck with the working with two people thing suggested above....it is how they teach objects to African Greys (think Alex and Me by Irene Pepperberg)