Sync relies on system notifications about file update to detect changes an upload updated file. Sync considers a file to be changed if its "modified time" or size change. There are cases when either if these is not updated or not noted. Manually "touching" the file so that its mtime changes will a solution here.
On any platform you need to open command line/terminal and apply the following command:
Windows: navigate to the directory with the file and run command
copy /b <filename>+,,
Instead of "<filename>" you need to put file name and path to that file, for example:
C:\Users\user\Desktop\>copy /b jira.txt+,,
Linux, Mac, Freebsd, NAS: