Random MP3s will not play on the iPhone music app

Certain MP3s that I have downloaded from my MacBook won’t play on my iPhone music app. if you select the song, it just won’t play. If the song is on the playlist it will be skipped. This can be completely random. If I remove the song from my computer Music app (1.2.5.7) and then add it back and re-sync. The file might then work. This definitely happened with an update because from about two years ago this never happened. It certainly never happened when the app was called iTunes.These are happening to MP3s that I have purchased outside of iTunes. I feel like this issue is not being attended to, as the priority is for streaming. Which I don’t want to do. I don’t know if this issue is being caused by the music app on the phone, or on the computer.Is there a resolution for this?