Why does music added manually or via iTunes not play

I have added music via my computer and garmin express to my watch. The music shows up on my watch. But when I try to play it from my watch it simply does nothing.

i previously had Amazon player installed which worked fine until Amazon decided it did not want to play the songs in my play list but rather play similar songs instead! I was using Amazon prime music and do not want to upgrade to music unlimited for £14 a month, when I already own the music I want.

so, I simply want to download 30 or so songs from iTunes. They are not listed as protected, and in any case I took the protection of one a converted it to mp3 and it still did not play. It was effectively an mp3 sound recording.

any ideas? 
does the watch need some player installed to play the songs? Or does it have its own native player?