To add to this thread, for historical/tracking purposes:
https://lemmy.world/comment/193148
A Lemmy user can not reply to my reply made with a Kbin account from the Kbin instance readit.buzz.
However, my reply syndicated on the lemmy.world fine. And his disconnected reply syndicated to readit.buzz fine.
It could be, as others have pointed out, delays because there are a lot of federation requests. Or, it could very well be a bug, and in this case, it seems to be coming from Lemmy.
Whichever is the case… just documenting it here for future reference.