Hello @LWVCT ,
Unfortunately, we're not showing anything in our logs that would indicate the exact reason these posts might be failing intermittently. With that said, we are aware that Facebook, and by extension Instagram, will occasionally refresh their APIs and permissions. This in turn will sometimes require new logins / reconnections from the user, and can potentially cause interruptions for some posts that would go out during these downtimes.
Our devs are working with FB to pin down if this is the true cause of the sporadic interruptions in successful posts or if something else is going on, so I've tracked the relevant service request to your account. If and when there's an update, they'll make sure to notify you.