Before giving my answers, I would want you to review the issue by clicking on the CTA link where the problem exists.
As per the recent changes, the requirements for eligibility to monetize the channel contents are 1000 subscribers, 4000 watchtime, and organic contents.
The possible reasons are:
1. Some one has reported your video because there are many reasons.
2. You might have deleted some videos after monetization and results in drop of watch time.
3. The title, description and tags are against YouTube policies.
4. Copyright infringement: Audio, background loop music etc. The copyright author might have claimed his or her copyright content on YouTube without your knowledge.
If none of the above is correct, please escalate the issue to YouTube support team stating your problem precisely attaching the link, screenshots of that video by signing in to YouTube from your desktop> Creator Studio> Reach Us page. They will revert back in 24 hours.
Hackers never learns but always wins!