How often specific Windows computer data is scanned depends on your item intervals, a feature documented
here. The default interval for OS information is 3 days, but someone may have changed your configuration. To have OS information updated, either change your intervals or click the Assets link at the top of the web console, tick the checkboxes in front of the problem client machines and hit the Rescan button on the left. The Rescan buttons ignore item intervals and rescan all Windows computer data.