Version with check-in/out is great but not always preferred, and certainly not infallible. As learned on a project a few years ago that recently hit again when a new machine was not similarly configured. SharePoint Designer has a dirty little habit of caching files, including say the XML/XSLT, JS and CSS you might be working with on your site.

No need to fret though. Buried in the settings of each version is a toggle that will ensure SPD never sets you back by loading a cached file.

Close SPD if it is open > Open My Computer  > Click the address bar > Paste in: %USERPROFILE%\AppData\Local\Microsoft\WebsiteCache

Delete everything within this location. Click the address bar  > Paste in: %APPDATA%\Microsoft\Web Server Extensions\Cache. Delete everything in this location

SharePoint Designer 2010 and 2013

Navigate to the "File" menu and then select "Options" > "General" > "Application Options".

On the “General” tab, under the “General” heading, uncheck “Cache site data across SharePoint Designer sessions”.