When QuickBooks American Express not syncing, it means there is an issue with integrating the American Express account with QuickBooks software. This problem can occur due to various reasons, such as incorrect login credentials, outdated software, or issues with the connection between QuickBooks and American Express servers. Troubleshooting steps may involve verifying login credentials, updating QuickBooks, checking internet connectivity, or contacting American Express or QuickBooks support for assistance at 1-855-856-0042 .