I've been having this issue with MSN for a while, where some of my contacts never show up online, even when I know they are online, and I show up as offline for them too. It only does it for a couple people, the rest in my contact list work fine. Messages don't go through at all either.
I've tried restarting, re-installing MSN, blocking and unblocking, making sure both our statuses are "online", and re-adding the contact. I've used different programs; MSN on my PC, Adium on my Mac, as well as the web messenger Ebuddy, and they all do it, so I doubt it's any of the programs themselves. I've tried looking up a solution online but couldn't find anything myself, so I thought maybe someone here might have another idea.