@Yasser_Alsawy I would suggest enabling TRACE level logging. The filewatch library that the file input uses will then log when it finds the file and whether it sees any unread data in it.
In theory using /dev/null on WSL was fixed to work properly back in 2016 (as you might expect, a non-working /dev/null broke a lot of things), but if the trace logs indicate it has already read the file then try changing the sincedb_path.