Why don't I receive all return messages on a group message in the same thread?

[AC Question]

I am having this weird issue where I have a group message with 3 other people. When 2 of the 3 respond all of our communication appear in the same thread. However, with the 3rd it never appears in the same thread. What makes it even weirder is that the 3rd is my wife's phone. We are on Verizon with the other 2 with a different carrier. All 4 are using Android phones. The problem persist whether I am using the stock messenger or a 3rd party app (handcent). Does anyone have an idea? Thanks