I'm using Autosync (same as dropsync) to synchronize Joplin in Android, in Linux I use file system sync too.
Notes and notebooks created in Android go fine to Linux.
Notes created in Linux don't go to Android.
What have you done exactly? I believe that just touching the files, or in other words changing their mtime to the current time, may not be enough, because there is no guarantee that Joplin processes only some of them in between, and then the rest will already have a date older than the recorded one.
In my experience, only setting the date to the future is a bulletproof method to force Joplin scan through and sync everything into the database. The future date makes it 100% sure that nothing it left behind.