We had exactly the same problem with Pebbles. He was great at recall but as his confidence grew so his response to recall diminished
We use a harness with a 30' line which we leave on him when he goes for a walk. This serves 2 purposes. Fisrtly, it's easier to grab hold of if he runs after another dog as we just step on it but secondly, we practice recall holding it. If he doesn't come we give him a gentle tug as a reminder and then reward him when he comes back. We've also all decided that the word "Here" is his recall word and we all stick to it.
So far the results have been fantastic
He even comes back when called if he's with other dogs. He can be reluctant sometimes if he's found a particularly good smell but if after 2 calls of "Here" we get no response we go to the end of his line and again give him a gentle reminder.
We're now at the stage where we're starting to trust him without the line on and he's still good - in fact we're really proud of him. I'm sure we'll get a relapse but we'll just go back to the line again for a short while
Hope this helps