It looks like it's been awhile since your post, but I wanted to answer this question in case you were still wondering!
Sometimes artists or their record label will choose not to release specific songs from an album that does have songs on Spotify. This could be for a variety of reasons. However, Spotify is adding music every day, so it's possible that these songs could become available in the future.
Here's a good post from last year on the topic that goes a bit more in depth. Hope this helps!