Paypal with a2billing

Hi there, i have paypal integrated with a2billing everything was working fine except the other day customer pay by paypal then the payment didn't add it to the customer account in the a2billing which it was don't before im not sure what happened all i need to do is enable that feature again so when customer pay by paypal and the payment goes through the credit so be applied to his account without me adding it manually anyone can help it should be easy job as all the code is there but im not sure where to find it to fix it