What helped for us..
Dog had plenty of dive to run to a mark if thrown on park grass,or in light cover..or in situations where she just plain stepped on it.
She had trouble sticking with digging out a mark in cover, or where she might have mismarked it..
What we did..
Removed any correction.. (Collar use, bad idea)
Shortened things up a lot..
While on walks and with a surprise situation, (get dogs attention with "hey hey hey") I would throw an orange bumper into heavy cover. Very close to us. Far enough she could easily see the throw ,but it landed in deep cover... As she struggled to find it (She would) I would encourage her to keep at it with "Hunt it up".
You want to have the dog have to hunt,,but at first its important for dog to have success with hunts of bumpers thrown in really deep cover, Knowing there is a bumper in there.. (Start off easy,,maybe moderate cover)When retrieved, give praise..
GRADUALLY increase level of difficulty, and depth of cover, and the amount of encouragement you give to keep the dog hunting for it.. Remember to focus on getting a good hunt, with the success of the dog finding the bumper.. increase difficulty gradually..
Eventually, because the dog has had success, HUNTING, we noticed marked improvement..
Once the dog gets really good at this,,, then you can add stuff like "Dirt Clod drills" or various marking drills.. (Y drill for example) alternating cover depth from light to deep, to keep dogs desire to hunt up..
Eventually on longer marks,, BB help can be an asset..
A good discussion about correct (opinionated) way of how a BB should help.. Different ways , different trainers..