Post didn't go out overnight, how do I find out why

Check in this order, it's almost always one of these four:

1. Connection expired. Settings → Connections. Platform tokens expire, and Facebook in particular expires them when you change your password or revoke access from Meta's side. An expired connection shows there and re-connecting takes about twenty seconds.
2. 48-hour duplicate guard. If the pool is small, every candidate video may have already gone to that account within 48 hours, so the slot skips rather than repeat. The post history will say the slot was skipped and why.
3. Empty or filtered pool. A pool that looks full can still have zero eligible videos if the campaign is filtered to a category with nothing in it.
4. Plan limit. Free accounts are capped at one post a day. The second slot won't fire.

The post history view on the campaign gives the specific reason per slot, including skips. That's the fastest place to start rather than guessing.