WooCommerce Amazon Pay issue

Hi,
Our website uses the WooCommerce Amazon Pay plugin.
Unfortunately, for the last few days, most of the orders are getting this error “Error: Order missing Amazon order reference ID.”
But some orders (less than 5%) are successfully paid for using Amazon Pay.
And from the plugin log, we found this:

<Constraints>
  <Constraint>
    <ConstraintID>AmountNotSet</ConstraintID>
    <Description>The seller has not set the amount for the Order Reference.</Description>
  </Constraint>
</Constraints>

Has anyone experienced the same problem?
Thank you for any help.