I have several videos with completely flat Impression graphs (meaning they haven't been given any impressions in many weeks/months), so I decided to try A/B testing new titles and thumbnails for them.
Once you select your alternative titles/thumbnails and hit save, your Studio will immediately say "A/B Test running" under every video affected - this made me very excited because, well, I guess I'm dumb.
I naively thought this meant YouTube will give these videos new Impressions to test new titles and thumbnails, but yeah, I was wrong. It's been several weeks now and no new Impressions have been given out to these videos, despite it still saying "A/B Test running".
So yeah, if you're like me and your videos barely get any Impressions after the initial push, make sure you set your A/B tests before (or immediately after) publishing your video. Otherwise it won't do anything.