Event names inconsistent
Funnel needs exact event names. Case sensitivity, trailing spaces, typos all break it. Naming audit fixes most issues.
Missing event parameters
Conversion events often need parameters (value, items, etc.). Missing parameters break downstream reporting even if the funnel step fires.
Device/session attribution
Cross-device journeys break funnels. Authenticate users where possible and use User-ID. Anonymous funnel analysis is increasingly noisy.