I've had Charlie for just over a week now and initially he didn't get it he just thought I was talking to him and that nothing was required of him. I switched to a whistle rather than voice as he ignored the word commands and armed myself with smoked mackerel and suddenly I'm his best friend and he knows what the whistle means (he will even come back randomly and sit on the spot so he can get a piece!). I discovered it was his must have smelly treat when I got a pack out of the fridge. A bag of very small pieces and only used for recall. Smelly, tasty, quick and according to Charlie much better than chicken
I always ask for come back and sit and then similarly to Sue vary the next step sometimes on to a lead, sometimes straight back to running round and sometimes a big fuss so he never knows what's coming next after the treat. I guess if you already have a pretty good recall a change to a whistle or new word and a change in rewards might work.
Charlie has learnt very quickly, however he's only 5 months so in a few more months when kevin arrives.............