If your user made the payment and after 3 confirmation if their invoice still getting unpaid and nothing stored in gateway log than follow the steps to check :-
Point 1). One of the reason could be your Server ip got blocked in blockchain server.
Point 2). If you are using cloudflare than disable the cloudflare temprairy and after test.
Point 3). If mod security is enable on your disbale that temprairy and than test it.
So hopefully your problem should be resolved.
