Why are my changes not saving in the document properties in a subsite in SharePoint?

Verify that you are a member of the SharePoint site with adequate permissions to save changes
Check the Items tab on the ribbon at the top of the document for any settings that might prevent your changes from saving
In the File tab on the ribbon at the top of the document, select 'Version History' to look for any undesired changes
Run a check on the document library for any problems like errors with content types that could prevent your changes from saving
Run diagnostics on the document library or site; this will identify any errors or issues that could be affecting your changes