cron job || it works on localhost, not in production

Hello,

I’m working on a plugin with use a cron job.

function fftp_activate_cron() {
    // Schedule an action if it's not already scheduled
	if ( !wp_next_scheduled( 'scheduled_task' ) ) {
	   wp_schedule_event( time(), 'hourly' , 'scheduled_task');
    }

}

The task seems to work perfectly on localhost, in view of the classic cron job in wordpress: on page request. But the same plugin doesn’t work at all in production. I have tried on 2 different severs.

How is it possible?

I installed the plugin Advanced Cron Manager to understand:

> on localhost
at the beginnig, i get as ‘next excecution’: At next page refresh
on a page refresh, i get the right information: In 2 minutes

> on production
i get: At next page refresh
even after several page refresh

Could it be a wrong init of the cron job which could explain the problem on production server?
Could it be a miss configuration (wp-config.php, server, etc)?

I am lost.
Any idea will be appreciate.

Thank you in advance,
jb