Playlist is missing songs

I have the same problem on my Fenix 7x. Can anyone help? Many thanks.

I am wondering are they all the same format?

Yes, they're all MP3. They're included in Songs, even can be played. It's just not in the playlist.

Exactly. The songs are properly uploaded on the watch in All Songs (and are in the right playlist in Garmin Connect), but only about half of them appear in the playlist on my watch. Very frustrating because this means I can only have 1 playlist on my watch.

This is a few months old, but I ran into this exact issue and recently found the solution. On my Venu and songs appear in the list of all songs, but were missing from my playlist. Several songs in my playlist were by a musician who had an umlaut in his name. I changed "Göransson" to "Goransson" and the songs appeared.