If a shipment didn't push from Shopify to Amazon, the order shows a Failed Shipment error in the app. Fixing it is a two-step process: correct the tracking details in Shopify, then retry the shipment from the app.
Note: Make sure shipment syncing is enabled in the app first. Go to Settings → Order Settings → Shipment and click the enable button to turn it on. Without this, the retry won't push to Amazon.
Open the order in Shopify and confirm the fulfillment has the right values:
Save the fulfillment so Shopify holds the corrected details.
The app re-pushes the shipment to Amazon. Once Amazon accepts it, the order's status moves to Shipped.