Some of my subreddits keep disappearing. Why?

reddit picks 50 subreddits at random (or 100 if you have reddit gold), and displays those on your frontpage. Every 30 minutes, it picks a new random 50 subreddits. That's why some subreddits come and go.
When you view the 'MY SUBREDDITS' dropdown or the 'YOUR FRONT PAGE SUBREDDITS' list, you are seeing only the current 50 selected. The top bar also only displays the current 50 subreddits, along with some additional padding subreddits. The only place to see all the subreddits you are subscribed to is here.
At the top of your list of subscribed subreddits in the sidebar 'multireddit of your subscriptions', click that to see a multilink of all of you subscriptions.
If you wish to shrink your front page consider creating multis and unsubscribing from a few subreddits.

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