Tag does not fire. Check your trigger condition matches the event or page URL exactly. In GTM Preview, look under Tags Fired. If it sits under Tags Not Fired, inspect the trigger evaluation.
Orders missing from Idealo reporting. Verify the Partner Code matches your Idealo partner dashboard. On a mismatch the pixel fires fine but Idealo discards the hit.
Basket items empty or malformed. Confirm your items variable returns an array of objects with the expected keys. Use the Variables tab in Preview to inspect the resolved value.
Click ID missing. The idealoid parameter only exists when someone arrives from Idealo. On direct or organic visits it is empty, which is expected. If it is missing on Idealo-referred visits, check that your landing page preserves query parameters through redirects.
Currency falls back to EUR. If you sell in several currencies, make sure the Currency variable resolves to the right ISO code per transaction. Empty or undefined falls back to EUR.