Would you mind walking me through the steps you follow to schedule a post on your site?
Do you use any plugin to create these posts, or to force the publication of a scheduled post, like the WP Missed Schedule plugin?
Actually I have recently installed WP Missed Schedule plugin (after a few posts missed schedule). However it is not affecting all posts, for example yesterday I had 2 scheduled and only on one of them Publicize didn’t work.
Publicize is only triggered when the scheduled post gets published without any intervention from the WP Missed Schedule plugin.
If Cron doesn’t work properly on your server, your scheduled post doesn’t get sent. That’s when WP Missed Schedules comes in, forces itself first in load order, then runs on init at priority zero.
Publicize (and other plugins modifying posting behaviour) won’t have a chance to load before this plugin runs.