The selection of songs for Premium and Free members is currently the same. Missing or unavailable music is almost always a decision by the artist/label to withhold the content from Spotify. Sometimes it's temporarily unavailable while they sort out legal issues or correct metadata, in which case it'll be back up as soon as they resubmit it to Spotify. While it might be a good idea to contact the artist or label directly and ask why the music isn't on Spotify, you can also fill out this form to let Spotify staff know of your interest. You might also be interested in supporting this idea, which suggests a simpler method for requesting albums missing from Spotify.
Please let me know if I can help with anything else! Happy listening 🙂