Tricky one. I personally believe a puppy should be off lead as soon as possible, and since allowing my pups off lead ASAP, I have found it easier to teach a reliable recall (and by reliable I mean I'd bet a months wages on my dogs recall in any situation)

However, if your pup is able to run up to another dog, even once, when you are calling it back, and it ignores you, that has reinforced that ignoring your recall is rewarding, and puppy is likely to do it again. And again, and again.

What I do to get a good recall is a bit of both. Longline and free running. I have the pup 'loose' when I know we are in a safe area (beach, enclosed paddock etc) and way from other dogs than my own at first. Periodically call the pup to you, (set him/her up to succeed, so wait until they aren't busy) and when they get to you, give a treat (I always keep something really special just for recall) and let pup go immediately. Call pup back at least every 5 minutes, if not more. Every time your pup comes back to you, gets a treat and then is let go to get on with whatever they were doing, you are sending the message 'coming to me when I call you means you get a tasty treat, and then get to carry on whatever you were doing'.

I would try and set up a meeting with another dog if you can. Have pup on the longline. Still keep calling pup back, letting go again after a treat, but on the longline this time.When your pup has seen the other dog, call him/her to you. They will probably ignore at first, so be patient. When pup does come to you, give him/her a treat, then let them off the line to play with the other dog. (hence why a pre-arranged meeting is best) When pup is tired/bored, have the other owner 'catch' their dog, and then call your pup. Pup will probably try to follow the other dog, so it's important the owner doesn't allow it to play with your pup any longer. When your pup gets back, give him/her a treat, then let him/her play with the other dog again, and so on.

That's only the very basics, but the idea is that your dog starts to think that in order to gain access to other dogs, they have to come to you first. They think you control their environment.

By letting thme off lead ro longline when as young as possible, they are far more likely to leanr a goo drecal by coming back to you, asthey will be too nervous to go too far and will still find you rather interesting. When they get older, they realise that you are not the most interesting thing in their world, sadly!