Diagnose with watch time rather than views
Views are the output. Watch time is the input. Open your analytics and chart average watch time and completion rate across your last twenty posts. If those fell before views did, the content is the problem.
If watch time held and views fell, look at posting cadence, account changes or a content category you drifted into.
The usual culprits
Hooks that got comfortable. A format that worked for three months and stopped being novel. Reposting content the platform has already seen. Posting less often, which reduces the chances of a hit.
Also check whether you started adding links, watermarks from other platforms, or heavier branding, all of which correlate with weaker distribution.
The fix
Return to testing. Three genuinely different hooks per week beats twenty variations of the one that used to work. Reach recovers through new formats far more reliably than through posting more of the same.