Why isn't my Facebook Pixel detecting purchases on the Thankyou page?

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Hi, So I try to install Facebook Pixel Code manually (since the pixel is runs by agency), the pixel seems working fine until it reach the thankyou page. So the agency detect purchase being made (because the purchase tracker is at thankyou page). But when we check it in shopify dashboard, it didn't detect any purchase... 1. What seems the problems? I put the purchase tracker in order status page. 2. The agency thinks that customer can access thankyou page directly without paying. If this is possible, do you have any tips to fix this?