Training a dog to come when called begins with understanding why a dog doesn’t come when called in the first place.
First reason a dog may not come when called is that the owner might not have the position of leadership necessary for the dog to naturally feel the desire to follow the command or any command for that matter.  Dogs have very predictable innate customs and asking a dog to follow the command of someone who does not have the correct relationship with the dog is simply trying to fight against mother nature.

The next reason a dog doesn’t come when called is because of a lack of motivation.  Ask yourself, “What reason am I giving my dog to come when called?”
Is your dog going to get something good for coming?  Is there a consequence for not coming?
Also, what are you calling your dog away from?  What will be his motivation to leave that situation or distraction?
Sometimes a dog has even experienced something potentially negative in the past with the act of coming when called.  For instance, being called over to receive a punishment, or to take a bath, have his nails clipped, or to be put in a crate. Of course it depends on how the dog feels about these things, but you should get the point.
Lastly, some dogs don’t really understand exactly what “come” means.  Unless, you take the time to teach your dog that “come” means specifically to stop what he is doing, travel a straight line to you, and sit and wait when he gets to you – it is only a guess as to what that word means to him if it means anything at all.

All of these issues are easily addressed by systematically following the Foundation Style Dog Training System.  Generally, understanding some basic concepts of learning theory, leadership, and how to train formal obedience in that order is all that you really need.