Can't put new songs in some playlists

[mygb98zf0lclwg6]

I am a Premium family user since last year. I am not sure from when but probably since then (I had an individual account before that) i can only download songs to one of my several playlists. I have tried deleting and restarting the Spotify app, downloading the song searching from the playlist I want it to be added on (I didn't see how to do this anyway) as suggested by other people but nothing. This happens the same on my Iphone and PC. Thanks for your help.