MixMonitor SIP recording issue

Hi all
I experienced a problem with call recording using MixMonitor and SIP:

  1. both sip users have nat=yes entry
    SIP calling sip -> only calling party voice is recorded
    SIP calling IAX - recording OK
    IAX calling SIP - only called party recorded

  2. when both sip users have nat=no, recorded wav is empty (only header 44bytes)

anybody any hints???