I'm having the same problem. Flexslider block appears great when logged in. Disappears when logged out. I've checked all the permissions... ARGGH!

I was having the same problem with Flexslider.

After checking all the permissions issues, I remembered that the content for my slides was "unpublished". I switched it to "Published" and, VOILA! It worked.

It's a common practice to have the slides "unpublished", so I don't know why this makes a difference, but it did.