Sony a7s and Ninja Star problem

[giostrante]

 Hi guys, I've just got an Atomos Ninja Star external recorder to use it with my Sony a7s but I have a problem and I can't find anywhere a solution. The record functions work fine, I can trigger the recorder from the camera button, it records with no problem on the Cf card but when I press the camera playback button to see what I've recorded I got a black screen. When I press the play button on the recorder I can see the thumbnail of the clip recorded for less then one second on my screen, then again black. How could it be? I am using an Atomos hdmi cable, the files are also recorded in XAVCs on the camera card, so I am supposed to see them, than why the black screen? Please help me, if you can link me a full sony a7s setup guide would be perfect too, if there's a solution inside of course. I hope someone out there is using my same setup and tell me a solution, would be awesome, thanks.