If you go into the Learn To Play section, click on the "VIEW ALL ARTISTS" button. Some songs are not showing up for some bands. Example...

Click on Doobie Brothers. Only one song shows up.

HOWEVER.

If from the websites overall search I type in Doobie, I see a search result showing 5 songs.

Seems like I've seen other examles of this, but didn't write down what they were.