Payment Errors, Payments Pending, URGENT

URGENT. I’m losing money and customers. Customers are unable to complete orders with (PayPal Advance Payments) PayPal or Credit Cards. I have had unpaid orders for a while now and I have been wondering why. Finally a customer replied and said when they tried paying with PayPal it would not let them and when trying the Credit Card method it gave an unknown error and just kicked them out. This has cost me hundreds of dollars with every order that has been processed but not paid. ALSO, I am getting orders that customers do complete and pay but they still show up as pending payment in my orders which I have to manually change to paid (and sort through with the ones that failed). This also does not change my stock quantity so I have to manually change my stock quantity and risk overselling products which I have already done. Please help quick.