We originally added fwbackup (previously: bak) files to safeguard against a problem that Freeway files could become unreadable. This was in an early version of Freeway, many years ago, and we replaced the original document format with a more reliable format in later versions of Freeway, but it may help to explain the following behaviour: backup files are not created every time a document is saved, but only when it is saved for the first time after it has been read. In other words, Freeway creates a backup file that it knows is readable: the backup file is identical to the file that it previously read.
So: if you close and reopen a Freeway document, Freeway should create a new backup file when you next save the document.
If that isn’t what is happening, it might be worth checking that the backup file isn’t locked.
Having said that, I’d recommend that you have a more effective backup strategy than relying on fwbackup files. If you’re not already doing so, it’s a good idea to set up Time Machine so that it backs up onto an external drive.