Hey,
Could you share the error message that your system gets when someone tries to make the payment (your developer would have shared it with you)? This will provide a clue as to what may be wrong. I actually tried creating a dummy payment and the site only gives a message Payment processing failed. So, in the backend, the error code must be available. Can you send a message to me over chat window and we may discuss the details? Once I am able to log in to the backend and see the error, I would be able to fix it for sure.
Thanks and Regards,
Rishi
(Automyze Consulting)