Play doesn't work with screen off, not sure why. What I did to bypass this was set up a task to unlock my phone -> turn the screen on -> launch Spotify -> play -> turn off screen. I used the secure settings plug in to be able to unlock the phone.
I found on my phone the Play intent wouldn't work if Spotify was still loading. Adding Task->Wait for 8 seconds after the Launch App command gave Spotify a chance to load and get ready before the Play command gets sent.