Shoutbox

Happens to me Quiet often, Sometimes a simple Signing in and out of you and your friend fix the problem.

Sometimes its due to Server Issue on M$'s side.
Like last time it happened to me it was around the time when people weren't allowed to sign in(contacts list can't be achieved and stuff). That was few weeks back.

So not really your problem. It M$'s fault. Sometimes your invite will get throught and contact accepts it but on your side still looks like your waiting for them to accept and nothing happens.