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Generally, incomplete payments means interrupted payment process. Below, you can find more detailed information about this topic:
As you referred, if you sure about the payments are made successfully, the only reason causing this issue is that IPN hasn't received to us. This is something you need to set in Paypal. Also, you will have option to re-send previous submissions.
To enable IPN for Paypal, just follow the tutorial from the following link:
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