I can't speak to sticking or freezing but I can speak to the "anxious" mouthing as I finally cleaned that up a few months ago (maybe less, Idk). Anyways, Kiela had her hold program started twice before I started and followed through with force all the way through. That was mistake number one. Mistake number two, looking through my log for her, was thinking it was good that she was adjusting the position of the bumper in her mouth or picking up as soon as she dropped with a better grip. At least I think that's a mistake because there shouldn't be any movement in the mouth. Someone correct me if I'm wrong. Mistake number three, that is STILL an ongoing battle, is people constantly "playing fetch" with her. And that means she brings the toy, stick, ball, whatever to the person, drops it at their feet, and they throw it over and over. She did well after she got out of force and I didn't start seeing problems until a few months later after intermittent training.
Her "diagnosis" was excitement anxiety. She thought the sooner she gave up, the sooner she'd get a retrieve. As soon as my hand moved toward her mouth, she'd start rolling/chomping. Any increased pressure and she'd just roll/chomp more.
Her "treatment" was a combination of a few things. We obviously went back to hold in the calmest and quietest setting and with both her and I very relaxed. I made her hold for a minute or so with my hand holding the bumper as well to teach her that just because my hand is there, she doesn't release until I give the command. I also would stand beside her, put my hand down, bring it back up, etc. Lots of praise ONLY for a good quiet hold. Now, with the positive comes the negative (sorry I don't have the techinical terms). Positive was the praise. Negative was a pinch collar correction for a bad hold. No, pop, hold. Gooooood (long drawn out said in a quiet voice). When I finally tried that it cleaned it up in 2 days. Also, a sit, nick, sit was introduced after the pinch as she doesn't wear the pinch in the field, only in the yard. And that helped tremendously as well. Indirect pressure?
What DIDN'T work was: chucking her under the chin, e-collar corrections before the above "treatment," gloved hand, bigger/different bumpers, etc. etc. I tried a lot before I finally got to this point. She will still chomp a stick during play fetch but I don't do much play fetch with her and she knows the difference now between play and work. I tell people to make her drop now and the only people allowed to play fetch is my little brother and sister and my dad really. She knows to hold properly with me when we're training. Today she had FF on birds and I didn't need any corrections for hold. We'll see how she does if I ever get her to a test or trial.
Just my experience. Right or wrong, it worked. The logic in my head was she needed a clearly defined consequence for not holding to show her she was doing something wrong, other than NO. AND it needed to be away from her mouth as that increased the anxiety. So it made sense to me anyways.