I am no expert, and nor have I had gundogs. But you may want to get purdys progress, as I am using the same techniques for the terriers too and it appears to be working, just a little and often. Retreival may not come natural to them so a nice toy may be good.
There is also a "tug" technique which some one sent me, I tink it was in contrymans. You may bne able to google it.
Good luck