Lansweeper pulls monitor serials from the client machine's registry. It is known that some manufacturers don't add the entire serial to the registry. You can compare Lansweeper's scan results with the results from the following monitor tool: http://www.nirsoft.net/utils/monitor_info_view.html
Most likely, MonitorInfoView will return the same information, confirming that the problem lies within the client machine's registry.
If, for monitors of a specific model, the *same* characters are always missing, you can contact support@lansweeper.com and we'll hardcode these characters into the Lansweeper service. We've done this in the past for several models. If the missing characters are different for each monitor however, there is nothing we can do to resolve the issue.