Short answer
Because the checkout finishes the order somewhere your tags do not run. One-click checkouts take the shopper off your theme, so the thank-you page where the Google Ads tag fires never loads. The order is real; Google just never hears about it. The fix is to send purchases to Google Ads from the server, using the order data and the click id saved with it.
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The symptom
The store is selling and the bank account agrees, but Google Ads reports a fraction of the orders. Or, just as often, Google Ads reports more conversions than the store has orders at all. Either way, Smart Bidding is now learning from a number that has little to do with sales.
Stores often get told this is simply how it is with a third-party checkout. It is not. It is a collection problem, and collection problems can be fixed.
Why the tag never fires
Checkouts such as Shiprocket, Fastrr, GoKwik and Shopflo complete the order in their own flow. Your Google Ads and Meta tags live on your theme and on Shopify's own thank-you page. If that page never loads for the shopper, nothing fires.
On top of that, ad blockers stop browser scripts, Safari shortens cookie lifetimes, and consent refusals remove more. Each one is a small leak. Together with an off-theme checkout, they are a flood.
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Why the number is sometimes too high instead
Over-counting is the other half of the same mess. The usual causes are a duplicate conversion action, a page view counted as a purchase, or a codeless conversion action counting button clicks.
On one store we found a single conversion action that had booked over 6 lakh fake conversions. Nothing new should be built until an action like that is removed, because the account will keep optimising toward it.
The fix: send the order from the server
The order still lands in Shopify, and on most stores the Google click id can still be recovered from it. That is what Google Ads needs.
We run a server-side Google Tag Manager container on Stape, on a subdomain of the store, and send each purchase to it from the store's order data. The click id travels with the order, so Google Ads matches the sale to the click that produced it. The checkout stops mattering to tracking.
We then trace individual orders through the Stape logs to the conversion action they landed in. A green tick in Tag Manager is not proof; a traced order is.
What a good result looks like
Not a doubled conversion count. Only orders that came from a Google click can be credited to Google, and on many stores that is around half of all orders or fewer.
The target is that Google's reported conversions land close to the orders Google actually drove, and not above them. On the store above, a clean measurement window came in at 105% of Google-clicked order value, inside a ±20% bar. That is a number the owner can finally trust, and so can the bidding.
Check this before you change anything
Compare a week of Google Ads conversions against the Shopify orders that arrived with a Google click id. Too low means the tags are missing orders. Too high means something is counting that should not be.
List every conversion action marked Primary. There should be one purchase action, not three.
Turn off auto-applied recommendations while you fix tracking, so Google is not rewriting bids on bad data.