Troubleshooting estimate invoiced amount

I created an estimate and subsequent invoices for that estimate.  The first 3 invoices show the correct amount paid to date, but the next invoice adds the first paid invoice amount to the 4th invoice amount and therefore shows the wrong 'paid to date' balance.  The 5th invoice then takes the first and second amounts paid and adds those, but leaves out the 4th invoice amount paid.

Can anyone give me suggestions as to what I need to do to fix this problem?

Thank you.