Hi Richard,
sounds like you're doing really well with her
I've come across this problem before, but not with my own working cocker, a friends dog did this.
There are two approaches or reasoning for this sort of behaviour in my opinion.
Either she is trying to 'play' with you on her terms and thus not returning the dummy to you and inviting you to chase her, which a young dog at her age is relatively inclinded to do.
Or she is wary of giving you her 'prize' and doesn't think that you've got anything better than what she's already got to exchange with her.
So in the first instance, I think the general idea is to go back to basics a bit and do some play with her (not with retrieves) so that you build up her love of your attention and that she really does think that you're the best thing since sliced bread. Then you go to do a basic retrieve and if she stops away from you then you turn your back on her. More likely than not she will come right into you to see what's up, make a fuss of her when she comes in close but don't take the retrieve from her immeidately (incase it is a possessive issue) Once you have her by your feet with retrieve in mouth try to exchange it with a tasty treat and give your command for leave it at the same time. She should release the retrieve ino your right hand and take the treat from your left. Once you have done this once successfully in this way leave it and do something else that she's got spot on like 'heel' or something and finish the short training session on a high for her and you. Then do another retieve the next day and then some other training work that she's good at and thus build up a positive and good raport with the retrieves. Slowly build up the number of retrives you practise per training session, but I would be tempted at her age to keep them short and few and far between because she's liable to have a few teenage tantrums etc... and it's better for her to have those when she's being asked to sit down rather than retrieve as the sit command is more easily reinforced properly again after her relapse.
If you think that she is wary of giving you her retrieve then again go back to basics and try to make yourself the bees knees in her eyes. Then do as above but have something of hers that she really loves, such as her favourite squeaky toy or a chew that she loves etc... when she stops on the retrieve make her aware of what you have on your person and coax her to come in towards you. if she does then make a quick fuss of her and give her a treat or a squeak of her toy etc... Then follow this routine and she should begin to rush right up to you in order to get her retrieve exchanged for something better. Then only reward every other retieve with her toy etc.. but always praise by giving her a stroke or a word of encouragement. Finally you'll be able to wean her onto just the odd treat every now and again.
I hope this helps
Your little one's a cracker, the very best of luck with her training
I'm just starting out again with training my new 9 week old dog pup
whereas Bella, his mum, is being geared up for the grouse again this year and is as solid as a rock
Best wishes
Mary