Training recall is the most important part of every dog training.
It is important to know that some dogs can be difficult to recall because they seem very interested in the surroundings (other dogs etc.). But this doesn’t mean that they can’t be trained to come when called for. You just have to train it for a longer time.
Start the training in a place where there’s nothing that can disturb the dog (in your house etc.). Give the dog a good treat when it comes or his best toy. In the beginning make sure that you only call when there’s a very good chance that it will come. Then you can tell the dog “this is what I want you to do!”.
After a while you can start training in surroundings with other dogs etc. Remember to give the dog a very good treat when he comes – and never yell at him after he comes. If you are not sure that it will come in these surroundings, use a long leash. Hold the leash and call – he will come sooner or later – give him the treat. Repeat the training again and again. For some dog owners it takes a very long time.