This isn’t a new problem so I imagine it’s either already on your radar from one of the much older threads or actually just a kbin/mbin issue and not a fedia-specific issue (I haven’t thoroughly checked the kbin issue tracker). In any case: submissions are wonky and either don’t reliably sort in queues (other than new) correctly and/or don’t reliably federate until some kind of additional action is taken on the thread. For example, leaving a comment or casting a vote (like upvoting my own submission and then retracting the vote).

Example:

• Test post created on another instance hasn’t federated after a few minutes.
• Adding a comment makes it federate as expected.