Its possible that you are sending a doAuthorization API Call then the status will be pending until you capture the authorization.
Here is a similar post that gives information on Express Checkout pending transactions. Both answers have detailed reasons why a payment is pending for Express Checkout:
Stack Question Pending PayPal Payments
So you either need a payment action of Sale or you need to capture your authorization.
From the PayPal Documentation to ensure that the transaction is completed you need to pass the following API Call:
PAYMENTREQUEST_0_PAYMENTACTION=Sale
You can find more information on the PayPal Developer Site:
PayPal Express Checkout Payment Information