When the file format is SOLIDWORKS 2015 or newer then the custom tab will not show in file properties. This is by design because SOLIDWORKS is now using the same schema for custom properties that Microsoft Office is using and you can see that docx and xlsx files no longer have a custom properties tab showing in Windows Explorer. This also applies to files located in Enterprise PDM or PDM Standard/Professional vaults.
Another scenario where this is seen when the file format is 2014 or older. If the file is held open by SolidWorks then the tab will not show.