A few possibilities to start with:
1. Turn on debug mode. Make sure the plugin is actually running the tasks when it’s supposed to. If it is not, it could be a problem with cron and you should talk to your host. If it is running the tasks, make sure that the queries it runs are correct (debug mode will log the queries.)
2. Install the WP Crontrol plugin to make sure your cron tasks are running correctly.
3. Check the plugin’s scheduled actions tab at wp-admin/tools.php?page=action-scheduler and make sure it is running the actions.
4. If none of that shows you anything, try without ACF first, just with core WordPress fields, and make sure it isn’t an issue related to ACF.
Certainly it might be another issue, but this may be enough to point yourself in a good direction.
Thanks a lot ! i’ll try this way, step by step !-)