Feb. 9th Update:
We're pleased to announce that the issue has been successfully resolved! Please ensure the mobile app is updated to the following versions: iOS 2.9.14 (349) & Android 2.13.7 (338). Thanks again for your patience.
Our Mobile Engineering team is actively investigating reports of tags not ranking in the mobile app. We sincerely apologize for any inconvenience and understand the significance of tag rankings, particularly in the mobile app. If you have our extension installed on a desktop or laptop, you may use our free Videolytics feature to find your tag rankings.
Videolytics appears on a video watch page on YouTube; go to one of your videos where Videolytics loads on the right, select the 'click to load' button and you can see if you rank for any tags used in your video: