After viewing the dog ringing the bell video i came to a thought....when my dog knows i have a treat in my hand she will not fetch, not matter what.
What do i do then? She just stands there looking at me waiting....and no i do not give in.
Food is a more desirable reward to her. I am sure lots of food motivated dogs do the same. From her point of view, why would she want to retrieve the ball if there is food available? keep the food away...
There are treat pouches, treat bags, etc. We used to use one. The one I liked best was on a webstrap that fastened around my waist and it had a spring that locked the opening in a stay open or stay shut mode as I wished. Now I just use a plastic bag in my pocket.
I think your dog has learned that when you have a treat in your hand, deliverance usually comes shortly.
My Puff learned that when my hand goes into the treat pouch or bag, food shortly follows.
I suggest you use something other than your hand to store and carry the treats.
its not that i always have a treat in my pocket; its that no matter where the treat is, she knows it exists, therefore she will not perform. I suppose all i have to do is not let her know that i have a treat until after she performs.
An interesting thing that some agility people use is a thick nylon treat bag that has a velcro'd opening and some handles. It can be used as a tug toy and also you can throw it as a retrieval toy. Hopefully someone on here knows what they are called or where to get them.
What I've been doing to ramp up Sophie's tugging skills is this:
Before she gets dinner, she had to at first nose bump the tug toy. I'd then put the food down. Once that was solid, I waited till she bumped with open mouth. next step she had to put her open mouth around the ball. Then I waited till she closed her mouth on the ball. Then waited for some resistance/tugging. After a week, we've got a solid minute-long, hard tugging..
How to translate that to fetching.. do the same up to the open mouth around ball on hand. Then have the ball on the ground and start again from step one. Once she is consistently putting the ball in her mouth, start waiting for her to lift it slightly off the ground.
Shaping is FUN! give it a try! You do need patience initially, but you'll be surprised how quickly a food motivated dog will work through a puzzle.