I’m not receiving msn messages, what can I do to fix this?

I'm not receiving any msn messages from one specific contact, even when they send me their half of the conversation later, which shows that they've sent it. This only happens with this one contact, which I find even more strange. This contact isn't blocked, because messages go through sometimes, although it's infrequent. This is extremely frustrating, what can I do to fix this problem? I'm using the 2009 version of windows live messenger.