If the only time you call your dog back to you is when you see something that you know the pup will run to or be heavily distracted by, the pup will soon get the idea and as soon as you call, he will start looking for the distraction, rather than coming back to you!!
Play Hide & Seek with the pup (only choose safe places to do this!) – for example, if the pup has become engrossed with something and isn’t looking at you, quickly zip behind a bush (make sure you can still see the pup but he can’t see you very easily, so that you can keep an eye on him), when he looks up and starts looking around for you, leave him a few seconds and then call him!
When he finds you, big reward, lots of praise! After a few times of doing this, you will find he will be less inclined to become too engrossed in other things and will keep his eye on you in case you disappear again!