Start with the action, not the dashboard
Write down exactly what each report calls a conversion. A lead form opened, a form submitted, and a qualified lead are three different things. Before comparing totals, make sure the reports are describing the same action.
Follow one complete customer journey
Trace the steps from the landing page to the confirmation. Look for missing events, repeated events, or triggers attached to the wrong step. Document what you expected to happen and what you observed, so the diagnosis can be checked rather than guessed.
Align the comparison
Check date ranges, time zones, attribution settings, and reporting delays. Different systems can credit activity differently. A difference in totals is a reason to investigate, not proof that one platform is broken.
Turn the finding into a decision
A useful diagnosis explains which decisions the issue affects. Can you still compare creative? Is the campaign optimizing toward the wrong action? Should a tracking fix come before a new test? Agree the change, verify it, and document what still cannot be measured reliably.