Just to confirm, have you already tried these steps: Tap the 3 dot menu > Not interested > Tell us why > I've already watched the video? Also, our Help Center resource has info on how to improve your recommendations: https://goo.gl/mwxH46 . Let us know if this helps!
Please send a feedback report directly from YouTube with these steps: https://goo.gl/WdI0BK . While every individual report may not get a direct response, we do review everything and use the details to improve user experience.