hmm I still have that issue with Millie at nearly 1 year!! and cockers are supposed to be retrieval dogs! LOL
She will come back probably 7 times out of 10 - when I was trying to get her doing it outside I would give her the occasional titbit when she brought it back - and I would say over and over "Millie Millie fetch", "Millie Millie fetch","Millie Millie fetch" in a really friendly happy voice - and she would just come back to me with the frizbee because of the noise I was making LOL
, then I would praise her and say "thankyou" (to make her let go- easier said than done!) and throw it again.
I didn't give her a treat everytime, just sometimes, then throw it again
Now when we play if she doesn't bring it back I walk away from her and then she chases after me with the frizbee so I can take it off her and throw it again
LOL - reading that back I sound mad .. which is probably true, my neighbours starting giving me a wide berth