First post so please feel free to ignore it
I had the same problem with my Springer, I used treats, I see that you've tried the same, with my Springer, if I used a boring treat he soon lost interest.
But have you tried it with a really tasty treat, I use to buy a large bag of cheap Iceland sausages for a couple of quid, pop them all into the oven, freeze them, then thickly slice a couple for training purposes reducing the slices in size as the dog progresses. but start off with thick slices, they cant resist them, shove them into your pocket and be sure to let the dog know where they are, I found that he would walk with his nose almost touching the pocket, give whatever heel command you use, and a slice of sausage when he gets it right, make him work for his treat, no freebies
Accompany this with lot of praise when he gets it right each time, but it has to be fun for the dog so would advise short training sessions, so he does not get bored and be consistant.
