Yesterday's AdMob estimated earnings are down 20–40% against your rolling 14-day average, and nothing obvious changed. Before you assume the market moved or open a support ticket, know this: a sudden AdMob drop almost always concentrates in a single breakdown. Your blended number fell, but the cause is one country, one ad unit, or one ad source — not everything at once.
AdMob revenue is just impressions × eCPM. So the very first thing to decide is which half moved. In the AdMob report, put estimated earnings, impressions, and eCPM side by side for yesterday vs the 14-day average. If impressions held but eCPM fell, you have a pricing/demand problem (sources 3–5 below). If impressions fell, you have a delivery/fill problem (sources 1, 2 and match rate). Knowing which half saves you 20 minutes of looking in the wrong place.
Then check these five breakdowns, in order. Each is a single dimension change in the AdMob reporting UI and takes 3–5 minutes.
1. Country
Break revenue down by country, yesterday vs the 14-day average. This is the most common and most benign cause. If your impression share from a low-eCPM geo (Brazil, India, Indonesia) jumped — usually because a UA campaign brought cheap installs overnight — your blended eCPM drops even though no single market got worse. Tier-1 countries (US/UK/JP/DE) earning the same per impression but making up a smaller slice of the mix is not a problem to fix on the ad side; it's a UA-mix shift. Confirm by checking eCPM within a fixed country (e.g. US-only): if US eCPM is flat, the "drop" is mix, not price.
2. Ad unit
Switch the breakdown to ad unit. Drops rarely hit every unit evenly. If one rewarded-video or interstitial unit accounts for nearly the entire fall while banners are flat, you've narrowed it enormously. Common ad-unit-level causes: a recently changed floor on that unit, a frequency-cap or placement change shipped in your last app release, or that unit's mediation chain losing a network (next item). Note the app version too — if the drop started exactly when a new build rolled out, the cause is in the app, not AdMob.
3. Ad source (mediation)
If you run AdMob mediation, break down by ad source. eCPM drops concentrate on one or two networks far more often than they spread evenly. If Meta Audience Network or a specific bidder dropped 50–70% on one format while the rest are flat, that source is failing — check for an expired API key, a disapproved/limited-ads status on that network's own dashboard, or a payment/account hold on that network. The test: temporarily remove the suspect source from one ad unit and see whether the others backfill the impressions at the old eCPM. If they do, you've found it — re-enable and contact that network.
4. Format
Break down by ad format (rewarded, interstitial, banner, native). Formats price very differently, so a shift in which format serves moves blended eCPM. Did rewarded impressions collapse while banners rose? That usually means a placement or reward configuration broke in the app — rewarded is your highest-eCPM format, so losing its impressions drags the blend down hard even if every format's own eCPM is unchanged. This is an app/SDK issue masquerading as a revenue issue.
5. Match rate (and the policy/serving column)
Finally, look at match rate (matched requests ÷ ad requests) and the serving status. A falling match rate means AdMob is receiving requests but not filling them — caused by too-aggressive floors (you priced yourself out of demand), a regional demand shortage, or an account/policy action. Check the AdMob Policy center and Ads serving status: a limited-ads or partial enforcement on an app cuts fill immediately and is easy to miss because earnings, not an error banner, is where you feel it. This is the one cause you must rule out before blaming the market.
How to read the result
Walk the five in order and you'll land on a pattern: mix shift (country) — usually nothing to fix; one unit / one source — a config or network failure to repair; format collapse — an app-side bug; match-rate fall — floors or a policy action. The mistake that costs people a day is staring at the blended number and guessing. The blend is the alarm; the diagnosis always lives one breakdown down.
Doing this by hand every morning gets old fast. That is exactly what we built Mediation One to do: feed it your AdMob CSV and it finds the per-country, per-unit, per-source anomaly for you and names which of the five buckets is responsible. The free audit is a single CSV upload — no SDK, no signup.