I've been through a situation like that in a very international team, where culture played a significant factor in how people communicate. Once people knew about their preferences, it became easier for them to collaborate.
Did you try to do some team building activities with the whole group? Sometimes getting them to connect on a more personal level gets people to open up.
Did you try to do some team building activities with the whole group? Sometimes getting them to connect on a more personal level gets people to open up.