iPhone problems when syncing with iTunes

I created a playlist on my iPhone and then synced the phone with iTunes, but two songs were removed from the playlist. None of my other playlists are affected and it always removes the same two songs whenever I try again. When I click on the playlist under "On My Device" in iTunes, it still says that the playlist on my iPhone has all of the songs it should have, but when I look at the playlist on my iPhone the songs are missing. I've tried putting the songs back in the playlist and resyncing, but the same problem keeps happening. The two songs are on both on my iTunes library and are downloaded to my phone. Both my Mac and my iPhone have plenty of free storage space, so I don't think lack of capacity is the problem.

My iPhone is on iOS 8.4, and my Mac is OS X Yosemite Version 10.10.4.