Unavailable local files on iPhone

[1170835482]

Can't understand why some local files work and others don't. Same origin folder same format. Why? It bugs me off! It just happens on my phone sometimes, my new iPod works fine, plays the songs that won't play on the phone (dark ones on the pic). It just shows a message about the sync (which I've done dozens of times)

It's a bug that haunts me for 2 years, and I've changed my phone 4 times in 2 years