Well... with mine, I took them out individually and put them on the floor. Once on the floor I would call their name and hold a treat out for them. First I would hold it close, then I slowly moved farther away from them. Soon enough, they got the idea.
You just have to be persistant, that's all. Good luck.
Every night before I go to bed, I call everyone to the cage door, and then take them out one by one for a bedtime yoggie. I hold them while they eat it and say their names to them. After about a month and a half, most of them know their names.
I have a certain tone for certain rats. Its soo sweet, there's 13 in the FN, and they often stick their noses thru the bars and watch me after settlign down. I call their names and they perk up and respond. When I am pattting large groups (the Horde of 13 for eg.) I pat them and say their name as I am, then I pat the next rat and say their name
treats are a great way to train them to come. BUT start off with healthy treats if you can.