Some NAS do not notify file changes/additions in a timely fashion. This includes the symptom of partially seeing some files but not the whole album in Roon. If you wait until the time indicated in Roon’s storage rescan settings, do the files then show up?
Since this is a source of potential frustration, I think the dev team is always reviewing this area to see if there are things they can do to improve things. But, as far as I know, there are limits because the issue is not in Roon but the NAS.
If you want the files immediately, then you will have to do a rescan. OR, you could move your files locally and just use the NAS as a backup copy. This is what I do. There are many threads discussing this if you want to give them a look: