Some guys when they are with their friends don't want to look like they are wimps following a girl around and then have his friends tease him about it so he may rather just stay with his friends instead of following you. Maybe the next time you see each other maybe ask for his phone and ask if you can call him. Nowadays it's okay for the woman to approach the man if they are taking too long to ask you out and he may be shy and afraid you may turn him down. You are doing everything you should be doing to show him that you are interested in him but he isn't taking the bait so to speak. You may have to make the first move if you don't want him to get away. He probably won't ask you out in front of his friends, try to get alone with him, but don't make him feel too uncomfortable. Remember, it might be out of that his comfort zone to ask you out, try to become friends with him, so it doesn't become such an awkward thing to do, you really need to signal him in to ask you out. The best way is to talk about doing things couple usually do, like going to the movies, going out to dinner and if you are game, mention the fact you have no guy usually to go with (try and do this in a light hearted way). He may be waiting for you to say something or ask. He may also be afraid of rejection so that is more of a reason to let him know that you are game for going out if he wants to.