MSN problem

For the last few days, many of my contacts have been shown as offline even though they are online. I tried to reinstall the whole thing but nothing happen.

Note: This is not because I have been blocked or otherwise removed from the contact list and some of my other contact can still be shown, don't know if this is somehow related to the mail tag but so far, most of my unreachable contacts are Yahoo (some other rarely online so it's hard to tell if this happen to them also)