it can be very difficult to determine the cause of a sporadic issue. If there was no db entry for the order and no user registered there might be something very specific to that order that is causing it to behave that way. Do you know if this was a new user or an existing user? The issue that I was referring to in my earlier post was that if you go through a second checkout you could have a situation where it is not recorded as a new transaction. This is what the code sample I provided should fix.
In the 1 in 5 ordes that did not go through did you get the email messages? If not what info are you using to determine that the order was not completed? Please post back with the details and any others that you can think of. If there is an issue with the code or how the pages are working it will be very important to be able to reproduce the issue so that we can determine what is going wrong.