Some local files won't sync

[darkknightank]

I have a weird problem wherein some of the local music files don't sync on my iPhone. I followed the usual steps (create a "Local files" playlist, add all local songs on there, and make it available offline on the device). Some of the files do sync (about 25 out of 36), but the rest of them refuse to sync. They aren't even shown on the device greyed out. 

The weird part is that a few days ago they were working fine. Any thoughts on what might be going wrong?