By the sounds of it you need to go back to basics.

Where most recalls break down is that, like you say, most dogs will come back if they aren't doing anything that interesting to start with and they think it's worth it because you have treats/toy etc. When it all starts to go wrong is when the dog is doing something that it considers more desirable than treats/toy.

What you have to do is start from the very beginning (a very good place to start, la la la) and get the dog SO conditioned that even when it's doing something fun, it responds to the recall command without thinking.

You need to pair a sound - a whistle is good; it tends to "break through" a "deaf" dog's ears more than a voice can - with something wonderful. Either treats or toy, or most likely both (depending on the circumstances - I find recalling a dog away from another dog is easier with a toy, you're swapping one kind of fun with another).

You need to start REALLY easy, in the house. Feed a yummy treat, while tooting the whistle. GRADUALLY (you can't really skip steps) work up to whistling the dog to you for a treat. Work up to calling from different rooms. Then from the garden when he isn't doing anything in particular. Then when he's slightly distracted etc.... don't make it harder until you're 100% sure that the step before is reliable.

Then progress to outside, whistling in low-distraction circumstances, GRADUALLY building up. If you aren't sure the dog will obey (say if he runs off after another dog and you haven't got that far yet) DO NOT WHISTLE. Either go and get him, or wait until he starts to come back THEN whistle.

Ideally, if you do it right (and it takes time but it's worth it) you will never have a failure. If you build up gradually enough, the response will be so ingrained that even in high distraction situations, the dog will turn and come back BECAUSE THAT'S WHAT HE'S ALWAYS DONE

It's all about pairing cues/commands/sounds with actions. A dog whose owner calls "Rover! Here! Rover! Rover! Rover! Here boy! What's this?? Here! Come on then! Rover!" when it's running away is actually hearing "Blah blah blah blah blah blah blah" - or even worse, is viewing his name/the recall word with "That noise she makes when I'm running away from her"