My free gift is not added

The free gift has two halves: the discount that prices the gift to free at checkout, and the widget that adds it to the cart. "The gift is not added" is a widget-side symptom, so this checklist starts there.

The checklist

1. The Stackable app embed is on in your theme, with its free gift setting enabled, and the storefront widget has been deployed. The gift auto-add does not appear in the theme editor preview until the app is deployed. Check the Widgets page for the live theme status.
2. The Free Gift campaign is live, not draft, scheduled, paused, ended, or blocked by your plan quota.
3. The cart actually qualifies: the threshold is measured on targeted items only, and any gift already in the cart never counts toward its own threshold.
4. The gift is in stock. An out-of-stock primary gift falls back to the campaign's next backup; if none are available, nothing is added, on purpose, so shoppers are never promised a gift that cannot ship.
5. The shopper is paying in your store currency. In a foreign-currency cart the auto-add stands down as a fail-safe (thresholds are set in your store currency).
6. The shopper visited the cart. The gift is added in the cart, so a shopper who never opens the cart or cart drawer does not trigger it.

The express-checkout caveat

Only whole-store and product scopes auto-add

The auto-add only resolves campaigns scoped to the whole store or to specific products. A Free Gift campaign scoped to a collection, a tag, or specific variants does not auto-add the gift, because that membership cannot be resolved reliably in the storefront. The discount still works either way: if the gift is in a qualifying cart, checkout prices it to free. If auto-add matters for your promotion, scope the campaign to the whole store or to specific products instead.

"The gift was added, then disappeared"

That is the widget doing its job: if the cart drops back under the threshold, the gift it added is removed, so a no-longer-qualifying cart never keeps a full-price gift by accident. It only removes gift lines it added itself; anything the shopper added by hand is left alone.

"The gift changed to a different product"

On a tiered gift with purchase, that is expected. When the cart crosses a higher spend threshold, the gift swaps up to that tier's gift; when it drops below, it swaps back down. The shopper always has one gift, the highest tier they qualify for, never two at once. If you instead see two free gifts in the cart at the same time, that should not happen with a tiered ladder; note the exact cart contents and amounts and report it.