Unable to Save or modify the part.
Look at the tabs for each open part that are located across the top of the graphics region.
The part that will not modify or save has a small image on this tab that looks like an 'Open Door' icon.
The open door icon, instead of the bracket icon for the part on the tab is indicating the user is in the Gateway application.
Switch to the Modeling application by selecting 'File-->All Applications-->Design-->Modeling' (or CTRL+M).
Once in the Modeling application, the part can again be modified and Saved.
Notes