Desperate for some guide for wp_schedule_single_event()

Hi
I am the author of the plugin – downML. It creates a ZIP file of the media library contents.

I am trying to implement offline zip creation using wp_schedule_single_event() call. But the functions that create the zips are not getting called (or something else)

How do I debug what the problem is? I am not using PHP class for this. Just a couple of functions.

Please please please someone give some guidance on how to do this.