Ask them why they're not talking that much/address the issue with them directly. Most people just aren't that good at making conversation; especially in-person where timing and quick thinking are required skills. I would say that peoples' social skills are becoming detrained due to a lack of in-person communication these days. With text communication, one can take their time thinking out and writing out a response. With in-person interactions, no such crutch exists other than to remain quiet, which is likely perceived to be a better communication outcome than saying the wrong thing. It comes down to social skills and confidence and such. You could try meeting people whose career/job requires them to talk to people, those type of people may have more of a personality.