some NAT and firewall configurations won't allow P2P communication even if UDP is allowed. much more detailed information about these kinds of cases can be found in this posting:
most likely one of your peers is behind a symmetric NAT and the other is behind a symmetric or port/address-restricted cone NAT. as mentioned in that post, you can use http://cc.rtmfp.net as a diagnostic tool to help determine why your connection might not be working.
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the top picture indicates a symmetric NAT (different translations for different far endpoints). the bottom picture indicates an address&port-restricted cone NAT. this combination will not be able to form a P2P connection.
if you can't make a direct P2P connection, you must use RTMP.
Not a solution, but maybe worth to mention: At least a reliable way to figure out if a stream is really working (sometimes it doesn't even if there is a NetStream.Play.Start event) is to let it play() and periodically check the byteCount of the NetStreamInfo object. It seems that this returns something around 270 for a surely working stream. Correct me if I'm wrong.