Group Policy can be difficult to design, implement, and troubleshoot unless you are fully aware of the foundational concepts that drive Group Policy with Active Directory. There are many moving parts with Group Policy, not to mention the reliance that Group Policy has on Active Directory functioning properly. When making changes within a Group Policy Object (GPO) in hopes for a desired outcome, only to have Group Policy not working correctly can be very frustrating.

many article is founded in internet about its problems. one of the best articles about reason for failing apply GPO is writen by derek melber. please check these links

http://www.windowsnetworking.com/articles-tutorials/windows-server-2008/Top-10-Reasons-Why-Group-Policy-Fails-to-Apply-Part1.html

http://www.windowsnetworking.com/articles-tutorials/windows-server-2008/Top-10-Reasons-Why-Group-Policy-Fails-to-Apply-Part2.html

http://www.windowsnetworking.com/articles-tutorials/windows-server-2008/Top-10-Reasons-Why-Group-Policy-Fails-to-Apply-Part3.html

thanks by www.windowsnetworking.com