The warning signs worth acting on
Beyond the backend comparison, four patterns signal broken tracking:
- A sudden drop in reported conversions without a matching drop in revenue or traffic.
- Conversion counts that look doubled, often two tags firing on the same purchase.
- Zero conversions from a campaign that gets clicks and produces sales.
- Numbers shifting right after a redesign, theme update, app install, or new cookie banner.
Each pattern points to a different failure: a tag that stopped firing, a duplicate setup, a broken link between platforms, or a site change that disconnected the measurement. The pattern narrows the search before any debugging starts.
Test the funnel directly
The definitive test costs one order. Place a test purchase on your own store and confirm it appears in each platform: Google Ads, GA4, and the store backend. A conversion that shows up everywhere proves the chain works end to end. A conversion that appears in the backend but not in the ad platform shows exactly where the chain breaks.
This test beats hours of dashboard archaeology, because it isolates the measurement from market noise. One controlled event either travels through the full stack or it does not, and either answer tells you what to do next.
Why site changes are the usual culprit
Most tracking breaks during site changes nobody connected to marketing. A theme update overwrites the snippet that fired the purchase tag. A new app injects its own scripts and conflicts with existing ones. A redesigned checkout changes the page the tag was listening for. A new cookie banner blocks tags it was never configured to allow.
None of these announce themselves, which is why measurement should be re-verified after every significant site update, not just at setup. The cost of the check is minutes; the cost of a quarter of decisions built on broken data is the marketing budget.