I didn't use to have this issue but recently while I'm for example listening to music on desktop and go to my phone so I can change the music it doesn't even show the music I'm listening to on my phone. I checked the "Devices Available" section and neither showed the other one. It literally looks like I'm logged in two different accounts on them. I can be connected to the internet on both of my devices and listen to two sperate songs.
Also if it helps I went to "Offline devices" page in Spotify website and it does recognize both of my devices there.