VisualSearch_UpdateCallback::QCAR_onUpdate isn't called

Hello,

i'm trying to implement cloud recognition into the ImageTargets Sample. My approach was to try and kind of merge the two samples together since I can't find sufficient information about which steps are required for cloud recognition and just looking at the sample codes didn't really help either. 

My specific question for now is: when and how is QCAR_onUpdate exactly called? The comment in the cloud sample code says 

// Callback function called by the tracker when each tracking cycle has finished

In my project this callback function doesn't ever get called, so my question is, when or how is this callback function called?

Thanks for your help.