Just tested an association where neither sub-league has playoffs, and it worked for me.
In your case where it didn't work, did all your leagues advance to the playoffs in their respective leagues? If one didn't, that would be the reason (basically the association playoffs will start when all the other leagues have crowned a champion). I believe before the last patch there was an issue where if you deleted a tiebreaker game, the league could get stuck in regular season mode, but I thought that was fixed in 20.5.46.
Otherwise, if you have the league that's stuck and not advancing, feel free to send over the files (
http://support.ootpdevelopments.com/...p-developments) and we can have a look to see what might be going on. Sometimes can be some weird setup issue that can cause problems.