Um, first since we don't even know why you can't play on multiplayer, our guess would be as good as yours.
when you don't provide any relevant info to help us help you, we can't do a thing really to help you.
2nd, there could be a whole bunch of reasons why you can't play multiplayer, your router or firewall may be blocking it, game needs updating, some other strange issues, its hard to say really.
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