Issues with Tower and Canon S90


I have just been trying to use a Canon S90 together with the android version 3.2.1 of Tower.
CHDK is running on the camera and I can trigger the camera via my RC (chan7) and via the mission planner software on my pc.

I started a “Structure scan” mission and selected Canon S100 (as there is no S90) and although the copter (pixhawk based) completed the flight, no pictures have been taken and it seems that the camera was not triggered. I am using the canon simple shutter cable.

What might be the issues ?
Where could I start looking ?