Missing or incorrectly loaded files?

I’d like some help and wonder if anyone can signpost me where to go? I’ve got a computer I use purely as a music server. It’s fantastic and I love it. I control it via the Real VNC viewer or JRemote. Normally it’s faultless but I have an issue. My music is showing under Audio, and under the Artist drop down it shows (unsurprisingly) the artists and furthermore, under each separate artist it lists each album. These show up perfectly on the JRemote so happy days. Unfortunately half of the music on my M: drive is missing? When I select Files, a little further down, and M:, all the artists are listed and, on selecting each artist, each of their albums. If I click on any artist or album, the music plays without fault but none of it shows up on the JRemote? I think there’s an issue with missing or unloaded tags? Sometimes I control the server purely from my iPhone through the Real VNC app, but mostly it’s controlled via an iPad using the JRemote for selecting and listening or the Real VNC app for editing etc. I want to get back to where I was and have all the Real VNC stored Files show up as Audio/Artist/Album which populates the JRemote. I hope someone can please help me.