eCart not writing back to database for a significant number of orders, despite successful payment at PayPal

For some reason, a significant number of orders are not writing to database despite successful payment, but the problem is that there is no apparent pattern to it, although I strongly suspect that there will be.

1) If payment at PayPal is successful, we've established that this is updated on either the confirm.php or pp_confirm.php page. Why would some orders be writing successfully to database, and others not?
We've investigated the following already...
i) Type of payment - PayPal Account or Guest Checkout - both are affected.
ii) New orders or returning orders - both are affected.

2) Does a user have to have a Website Payments Pro account in order for eCart integration to work? Our client has a Business account, and we have API credentials set up for that..

When an order doesn't write back to database, following successful payment, it's also not redirecting the customer to checkout_success.php, which we know because an e-mail should be generated at that stage, but isn't.

What I'm not clear on is how it's possible for an order to be successfully processed by PayPal but the Store Summary in Database to not work.

Hope you can help. FTP details provided below.
Thank you.
NJ