Depends on your location some channels might be blocked for non-premium subscribers.
For instance Adult channels will generally be available for premium subscribers (over 18) only. Same goes to some of the premium Sports channels.
If your subscription expired, those channels will disappear from your play-list.

Another reason for missing channels is is your were using our personalization option.
there are few types of personalization:
1. Fast personalization from the application
2. Auto personalization based on your profile in our Web-site
3. Manual personalization in our Web-site

If you used one of those methods, you might have caused some channels you need to be hidden. the fastest way to restore your channels in the playlist is to use "Fast personalization" from the Login menu in the app. And click on "SHOW ALL" in the personalization menu