There are currently over 43 million tracks and rising on Deezer but not all artists or rights holders choose to put their music on streaming platforms. We need to always respect rights holders' wishes so for this reason some music is not available.
If an artist has a large back catalogue with different rights holders controlling different albums you can sometimes find gaps in the catalogue if a certain rights holder has not yet agreed to allow the music they control to be streamed.
Streaming rights are also defined by territory. The tracks that appear in the search box when you are looking for music are the songs that are available in all territories. In a small number of cases a user in a different country may not have access to different tracks so an "unavailable in your country" message appears when they try and listen.
We are constantly updating and re-negotiating our agreements with rights holders and new music is being added every week. Hopefully anything that is currently missing will be available very soon.