Is it common to have missing bills in TimeSlips?

I have noticed the last few months more and more bills are disappearing from TimeSlips. Here are the steps I use to create a bill for a client.

I create a pre-bill which is saved as a PDF, then I create a bill, which is sent to the client. After the bill has been printed I usually scan the printed bill to the clients folder, but if I forget to scan the bill I printed I will go to the bills section and reprint the bill, which is where I also verify if the bill has been posted to the clients account. Nine times out of ten the bill is there. The one time I noticed the bill missing, it was an error on my part which I was able to fix easily.

Where I have been noticing the missing bills is when I go to post a payment to the clients account. It will show all past payments made but the unpaid bill will not show.

What am I doing wrong?