What version of MediaMonke for Android you use ⋮ -> About MediaMonkey?

What file type(s), perhaps the device doesn't support it. Have you checked with a file browser that the files are actually on the device?

Using a Samsung s22, although I've had the same issue on windows 11 too at times. Originally added music and synced with the app and worked fine. Added 2 new albums to my music file on the phone and they won't play. They show up but clicking play or on a song just makes it skip endlessly through the tracks.