YouTube is very strict with copyright. Once a video is flagged for copyright, YouTube will not promote it, and most of the views will come from subscribers or people who manually search for your channel or video.
The only solution right now is to reupload the video without the copyrighted content, but it might not perform well again since the original video has been up for some time.
Alternatively, you can try using YouTube’s editor to remove the copyrighted portion and see if that helps.