It sounds like she's trying to bury the ball as she sees it as very high value and doesn't want you to take it from her. My 7 month old does this with her yak cheese chew as we don't give it to her that often so she must hide it from us (often forgetting where she put it and hiding it from herself too
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Have you got a narrow corridor/hallway you can use to go back to basics? If you sit at one end and block the exit then you can stop pup running past and give lots of praise and fuss for her bringing it back to you (even if it wasn't exactly her intention...!) Or you could create a run using netting in the garden that narrows at the opening so you can physically stop pup running past you.
I'm sure you will have gone through all this or similar when pup was really young, but I'm finding that now my girl is in her adolescence we're having to go right back to her earlier training as she's starting to 'forgot' things we already taught her or is just pushing the boundaries.
Another ball hidden in a pocket is also a great way to get her back to you and drop the one she has in her mouth. I've been using a rabbit fur ball to get her to 'dead' a tennis ball if she refuses first time. Just show it to her and waggle it a bit, and if she drops 'good dead/drop, good girl' then back in your pocket quickly whilst you grab the dropped tennis ball. Our gundog trainer told us not to give her the rabbit ball or ever exchange a ball for something else as that makes the tennis ball even more valuable to her.
I never normally do more than 3 solid retrieves in one session as you want to end on a high, and I've found that any more than this and she starts monkeying me around! The joys of a teenage puppy!