I read that STUN is for non-symmetric NAT, and RTP server is for symmetric NAT. 

Supposed A calls B.

If A, B both use symmetric NAT and STUN, they cannot hear each other

If A or B use non-symmetric NAT and NOT using STUN, they cannot hear each other.

Why is that?

I read http://tools.ietf.org/id/draft-takeda-symmetric-nat-traversal-00.txt for Prediction Failure, is that related to this problem ?