In my particular situation with my current shepherd, she was tougher than the past two teaching the retrieve. I read up on the subject and one article ( which I wish I could find again ) described the "right of ownership" in a dog's world....or at least their interpretation. It described how dogs claim ownership to certain items and depicted all of a dog's body postures, positioning of claimed objects, reactions of the dog if it senses some other entity might be making a play for it's prized possessions and of course a dog's reluctance to relinquish such claimed items......it was a good read and helped me deal with the same problem you might be experiencing.....
I started small.....really small and slowly shifted the dog towards a direction where she didn't have 100% claim/rights to objects she guarded...including the objects for retrieval. I remember working in the living room ...a foot away from her and progressed from there...all that good cheery stuff upon successful completion..as small as it might be.
I also recall taking a position similar to how a dog might have "claim" to an object...whether a dog is laying down or standing tall over a prized possession with the object between their front two legs...they supposedly are letting the world know this object is mine....When my dog took this position with her prized tennis ball, I ( on hands and knees with head high..no playful bow posture on my behalf ) crawled over to her slowly but surely as she stood proud over her claimed tennis ball and lowered my head toward the tennis ball to take it.....I knew what most likely would happen and it did...she reacted to my attempt to dethrone her from "owner" of the tennis ball....and I gave her a response which squashed her attempt....yes, it was a perilous moment since her teeth made contact briefly with my head but I knew it was coming and as I said ...I stopped her dead in her tracks......After this one encounter...everything changed...there was little if any resistance going forward from her to claim ownership again..since this one instance laid down a new law of sorts that the ball was mine....and I don't mind sharing.
Anyway, I did this with my 4-5 month old pup...would I do this a full grown GSD or a dog I am not familiar with?...no way...I would have taken stitches most certainly and I counted on my pup making a half-hearted attempt to stop my advance for ownership....I will say, it was more formidable than I thought it would be. But I was dealing with my dog which already had a sense of "who's who"..but she was still vying for position at times with me and she was well within her rights to do as she did.
Would I suggest what I did to others without knowing them..such as in this forum...no way. I am simply describing what I did and here's why...I am a believer in dealing with a dog in a dog's "language" rather than expecting my dog to interpret my human "language" at many times. I have taken much criticism and joking at times for "acting" like a dog at times to get a message across to my pup at times...but that's okay...it has expanded my understanding. And yes, I know....the dog knows I am a human not a dog but at times "dogspeak" works wonderfully.
Sorry to burn your eyeballs with this novel...but it's a free forum full of free advice...and the old saying is pretty true at times.