Why genius does not create playlist with all the songs (i pod nano gen 7)?

Genius uses the file tags to work out if other users have played these files in a playlist.

If the file is tagged incorrectly or it does not match files that Apple knows about it cannot use these files.

Your genius data is synced from the iTunes computer, so you will need to correct any bad tags in iTunes & resync the iPod to see genius updates.

There should be an 'update genius' option in iTunes via the Store menu. If it is up to date then I'm afraid Apple doesn't have enough info about those files. Even files from the App store can fail.

Apple have been redefining the word 'genius' for years, it's still not as smart as you would expect 🙂