This is most probably because you downgraded the rival Fortress at the incorrect time. For downgrades of other Fortresses to count towards your own upgrade, the Requirement to downgrade a specific Level of Fortress must be shown in the Fortress Tab at the time of the attack.
The Requirement will not be considered complete if you downgrade an enemy's Fortress after your own Fortress was downgraded and the Requirement disappeared.
The Requirement will not be considered complete if you downgrade an enemy's Fortress after your own Fortress was downgraded and the Requirement disappeared.