PayPal Express Checkout frequently fails
I've set up PayPal Express checkout pages using eCart5/DW6. Sometimes it works fine - I get all the way through the checkout process and the cart updates my db.
Sometimes, however, I get all the way through the checkout to the last page of PayPal confirmation, which tells me I will be taken back to my site to confirm the transaction. Instead of taking me back to the confirm page, however, I'm taken back to my cart display page and the cart appears empty. The transaction doesn't process, and no record is created in my db.
I'm not taken to the pp_chechoutfailure page, so I don't believe it is a PP authentication/approval error.
It seems like perhaps the cart has indeed been emptied, or perhaps the order number didn't get set in the session variable, but with all the moving parts, I can't figure out why this is happening.
Any suggestions of where to start my trouble shooting would be greatly appreciated.