Songs not syncing to device from iTunes playlists

Some new songs that I add to iTunes playlists do not show up on the same playlist on my device. If I add the songs myself to the playlist it belongs to on my device, then sync to iTunes, it removes them from the playlist. I have no idea what could be causing this, nor do I have an idea of how to fix it.

I attempted to create a new playlist, transfer the songs to it, and sync that new playlist, with some success, but lately it is not working for the newest songs in my library. Any ideas for a fix?