If you are noticing that data isn't being synced with GP-Billing from the medical system correctly, for example, you are missing customers or bills, the most common cause is that the version of GP-Billing you have installed on your computer is outdated. In order to resolve this you will need to uninstall and then reinstall GP-Billing using the most recent download link. The download link and instructions on installing GP-Billing can be found in this article: Installation.