Well, without seeing her I would say its a combo of two things.
1. Force Fetch
2. Better OB when it comes to picking up the bird and retrieving it. Her job is not to stand over it and crunch on it. The second she finds the bird her next thought should be get back to handler. I would treat it just like shopping.
The problem will be harder to fix as its now most likely an ingrained behavior.
Black Ice Retrievers
Too many dogs to list (By some Bitch)
"Helping Hunters Train Their Dogs"
I to think that it is a combination of things. To much collar pressure on fetch ( Pick up ) and a nerves hold.
I just went through this with a dog I had in for break out training. First thing is that, this is not a issue that is easy to fix or I would not get so many of them.
Number one is to take care of the hold. put her back on the table and make sure she is perfectly still before you take the object and I mean still no movement. you do this by grabbing her by the scarp of the neck and pull up. This is not as easy as it sound, you might have to use both hands and do not take it until she is still.
Number two, Use walking fetch with a lead. ( No collar with this drill ) put down 4 bumpers spaced about 15 paces apart move to one and say fetch as soon as the dog hits the bumper pull up on the lead and say hold and put her in the heel position. if she get all nerves do the same thing that you taught her on the table. Also in this process I will teach then the no command. move to a bumper and say fetch and the next one will be a no. Do this until she has it then go to birds.
Hope I did not miss anything.