This just started happening about 5 minutes ago. I sent a message and didn't see it go into the conversation window so I sent it again. A friend respond asking why I said it twice. I asked 'are you getting this?' and they said yes. It is NOT showing up! However when I go into the feed to see ALL my messages to everyone I've talked to, you know where it says the last message that was sent or received? It does show there. Just not in the conversation window. Helps!