[Freeswitch-video] no data flow in a-leg in proxy media mode

hi, I am a developer from China my freeswitch(1.5.final version) is set 'proxy media' mode(light proxy mode without rtp header parsing), and the media has no audio but video from an IPC, i 'm succeed on sending video data from IPC to b-leg continuously, but for a-leg, i can't see any video data flow, hope anyone can help, thank you in advance! more info: for developing my project, I made some configurations for freeswitch to let special SDP passthrough fs, and in my case(linphone<--freeswitch<--IPC), there is no audio but video only(which means I have to do some hacking thing on freeswitch to let it runs normally).