Get rid of the retractable 15' leash and replace it with a 25', probally 1/4 or 3/8 cotton rope for now. The retractable leash has no finesse. In other words, it works like a winch. You only want to pressure her enought to get her going in the right direction. Learn to slide the rope thru your hand's and let it out, in smooth motions. It's your control and your immediate means to gain response. Remember from military dog training, timing is everything! There's lots of way's to get from point A to point B but if your timing is wrong, you create lot's of avoidable problem's.
On the tooth pinch thing, good to quit! What you don't want to do is associate anything unpleasant with anything in her mouth. Leave her mouth alone least she decide that the unpleasantness come's from the object in her mouth. If that happen's, the fix is forced retrieve training. Not an overly pleasant thought but very effective. In force retrieve training the dog usually goes down like a beat pup but, come's back in the end,IF IT'S DONE PROPERLY. Don't set her up for it.
If you have an old shot bag, it'll make a better retrieving buck than a sock. Also a wood dowel with square pices of wood at each end. The problem with the sock is that her teeth can hang up in the material. A shot bag cut down to size for her then stuffed tightly with kapok is a better choice that will later float. The wooden dowel with square blocks work's well but use the square's on the end. What they do is to raise the dowel making it easier for her to pick up the dowel.
If she'll play at retrieving with these things, leave the scent out. Likely from what you said she will. Remember she's young and she's learning. With her there's a lot of association going on. If she goes for the buck that smell's like a bird and get's slapped in the face with a branch, or whatever, she may well associate the slap with the thing she's picking up AND the smell of a bird. Retrieving is retrieving and it's much easier to bring the bird in later. I might mention that in force retrieve training, it is completely done with wooden training dowel's. When that is complete, the dog is then changed over to a frozen bird and finally to a fresh killed bird.
the purpose being to teach retrieving, not retrieving bird's. There's a difference.
Couple thing's to remember; Never set your dog up to fail, pay attention to what your training, don't give a command you can't enforce (check cord)
How about a photo of this pup?
Don
