Start with what kind of thing it was, because the causes do not overlap.
A social post
Is it still awaiting review? Generated posts are held until approved, and one left in that state does not go out late — it does not go out. Check the calendar.
Is the account still connected? This is the usual answer. See a connection stopped.
Did the platform reject it? A failed post shows which platform declined it and why. Most reasons are platform rules rather than anything about the content — media dimensions, posting frequency, an account type that cannot be posted to by any third-party tool.
An invoice reminder
Is the sending domain verified? An unverified sender is the most common reason a reminder was sent and never seen — it arrives in spam. See invoicing setup.
Is the invoice actually overdue by the number of days the rule uses? Reminders fire on a schedule you set, and "overdue" starts from the due date rather than the invoice date.
Does the customer have an email address on file? Reminders cannot go anywhere without one.
A review reply
Replies to reviews are drafted and wait for you. Nothing is posted to your profile until approved, so a reply that has not appeared is usually still in the queue rather than lost.
The check worth doing first
For anything scheduled, look at whether it is waiting for approval before looking at anything technical. It is the most common cause and takes five seconds to rule out.
If it should have gone and did not
Tell us what it was, where it was going, and roughly when — see getting help.