RPM isn't just a metric — it's a diagnostic tool.

When your RPM drops 20% in a week with no traffic changes, you have three places to look: fill rate, ad unit placement, and geographic traffic mix.

Fill rate issues usually mean your ad partner is struggling to monetize your inventory. Test adding a secondary network via passback.

Placement issues are trickier. An above-the-fold unit that's 90% viewable will always outperform one buried below content. If you haven't audited your viewability scores in the last 30 days, do it now.

Geo mix is often the silent RPM killer. A surge in tier-3 traffic from social sharing can tank your weekly numbers even if your core audience is unchanged. Segment your RPM by country in your analytics before drawing any conclusions.

Fix the right problem first. Most publishers jump to ad layout changes when the real issue is a programmatic demand slump. Check your CPM floor prices and compare with last quarter before touching anything.