Songs would not play consecutively in my iPhone X playlists. The basic problem, I found, was that I had inadvertently turned on Shuffle. The Shuffle key on the main playlist screen does not light or change in any way when touched, so I didn't know it was on. To fix the problem I chose a song to play; in this case, the first one on the list, but it can be any song. While it was playing, I touched the title of the same song at the bottom of the screen which took me to a different screen. Scrolling down a bit, I saw a list titled "Up Next." It showed the random order in which the songs would be playing. It was on this screen that I noticed the Shuffle key was illuminated. I touched it to turn it off, and the "Up Next" list immediately mirrored that of my Playlist, meaning that the songs would from that point forward play consecutively. This worked for me. I hope it helps you.