First of all, please check if the downgraded enemy Fortress matched the level specified in the upgrade requirements. These requirements can be found in the League Fortress window.
Also, make sure that when the battle at a rival Fortress took place, your own Fortress was on the appropriate level. Like in every other quest or event, the upgrade requirements must be present on the list to be able to fulfill them.
Whenever your Fortress level changes, the old requirements disappear since they are substituted with the new ones. If the downgrade requirement was not fulfilled, then it is most likely that you did not have it in your Fortress when the battle took place.
In other words, when you downgrade an enemy Fortress to fulfill the level 9 upgrade requirements, your own Fortress should necessarily be on level 8 at the time of the downgrade.
Nevertheless, you only need to complete the Fortress downgrade requirement once on each level. Therefore, if your Fortress was downgraded, you will not have to downgrade a rival League Fortress again to bring your own Fortress back to its previous level.