When I’m talking to someone sometimes the messages doesn’t arrive.
Did you use ink at all by chance? Just using ink once in a session will start a snowball effect that eventually stops you from being able to send messages, even many hours later. To fix this, you need to fully close (shut down/exit/end msnmsgr.exe process) and restart Messenger.
This annoying bug was present all the way until the 2009 version when it finally got fixed… and then the ink feature was removed in 2011 
