This problem is a bug in flexslider version 2.2.2 download package. Even the demo file in the downloaded package does not work fine, really!
After so much investigation I found out the "jquery.flexslider.js" has problem and got a working file from version 2.2.0.
The solution: Get a working package version 2.2.0. If you are interested make a comment to make a working package available to you for download.