A “scan” just adds new files, so that should work.
A “Full scan” completely wipes the database and rebuilds it from scratch, more or less.
What platform is firefly running on? Could it be that you are facing a permission issue? Meaning that firefly isn’t allowed to access those songs?
Maybe they are also a different codec, one that firefly isnt setup to scan just yet?
Either way, FireFly won’T automatucally pick up new files, unless you explicitely configure it to automatically rescan at periodic intervals, or press the “scan” button every once in a while.