SolidWorks does a crap job of telling you when you actually have write access to a .swp file. You hit save, but, as you experience, nada. Zip. Zilchro. No savey, no messagey.
As soon as someone uses your macro in their session of SolidWorks, it will be locked for editing. Good luck figuring out who has write access or how to get it back.
Your best bet is to edit a copy on your local machine and then ask everybody to please, please, please close SolidWorks and log out before they leave for the day. Then stay late until everyone is gone and hope you can replace the file.
Once you have done this, you might have luck marking the file as "read only" in Windows Explorer. Then you may or may not have an easier time replacing the network copy with your local copy before everyone goes home.
Your other option is to write it correctly the first time. I have never been successful with that one either though.
You can see on server who has write access to file. Then you can friendly ask to close it, or self close it on the server.