I understand that this can be quite a frustrating situation. I had a similar issue with my eldest. I was very concerned that he talked more to people when we were out. After trying to encourage him to talk and getting nowhere, I came to realise that it was OK for him to be quiet-that's who he is.
Some people are loud and talk to everyone, some people are quiet and prefer to take time to get to know you and that's my son. He's a watcher; he prefers to watch a situation and get involved when he feels confident to do so. Although this can be frustrating for me, it's the way he is. Pushing him to be something that he's not didn't help. Since I have backed off pushing him to talk, he is gradually becoming more confident and is actually doing the talking that I was trying to force him into. This was the situation for my son, allowing him to be himself has actually given him the confidence to get involved in coversations with people. It has taken time, but it has been alot less stressful than trying to push him into talking.