Some payments become after payment transaction the status -> Pending.
It does not matter wether paypal, payza or whatever you use -> the most payment gateways has a pending status!
Now we have the following problem - it is not certain that the payment would be completed successfully.
The pending payment to be classified as failed is even a bad solution.
This status has been simply forgotten in your script and the relevant payment gateway plugins.
My suggestion for this problem you can read in this thread - LINK
Solution: Not found.
====Unsolved====
//Steffen