My music keeps deleting itself

There're certain songs (and it's the same ones each time!) that keep deleting themselves from my iPhone library. However, when I bring up iTunes, under my phone it says the song is still there, but nowhere to be found on my phone. I have to constantly keep going back and re-adding the same songs each time and it's getting quite annoying. What's the cause/fix to this? (I don't use iTunes Match, and have unchecked 'sync automatically when connected, to no avail)