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Please do check this guide regarding incomplete payments:
Please do take note that clicking "Complete Submission" will not process any payments, this will just add the submission in your completed list. The reason why you are having incomplete submission is that users might have accidentally closed the browser when they are in the payment gateway or they changed their mind and did not proceed on the payment, this means that a transaction never existed.
To check if the submission has a payment, I believe you will need to manually check your submission list and cross match it with your transaction history in your payment processor (I'm guessing you have integrated Paypal).
Do let us know if you need more information on this.