so, when I trained ours, we got to a certain point, not bad, but not there really and me stopped makng progress.
I think it is because the stop start method works, but with a sniffy dog like a spaniel, there is a natural stop start in them, so, they tend to walk like this
not pull, quick tug ot lampost, no pulling, dart across to grass, no pulling, tug forward to nice smell.
He also did what a pp said, which is to stop as soon as I stopped, but keep the tension on the lead, which means you can stand there for ever waiting for him to relax the lead.
So to get that last bit in, I took the stop start one step further.
I stopped. If he didn't immediately loosen the lead, I took a slow step backwards. It never took more than 2 steps.
I also followed something I saw on-line which has worked really really well.
We have pulling hot spots, eg past a particular house/tree/lampost.
When he pulled there and stop start didn't work, I turned round and walked bakc down the road. When we were a fare enough away from the hot spot, I stopped, made him sit, rewarded him for sitting and then started again. Eveyr time he yanked towards the house, I truned round and wnet back to a [;ace before he yanked. Made him focus on me, sit etc rewarded him for sitting nicely and starte doff again.
It works. I have done it again recently when we changed our route and found a new 'hot spot' = a house with a dog in the garden. now he walks past it liek an angel.
One last thing, never ever allow him to go through a door/gate ahead of you, even in the house. if you walk through a door with him, tell hime to wait, you go first. Do the same when out.