In a nutshell, every admin must follow this flowchart given by Citrix to troubleshoot all VDA registration issues:
Note
For more details on the flowchart, please visit the Citrix article at http://support.citrix.com/article/CTX136668.
For basic troubleshooting, you can use the XDPing utility from Citrix that provides very good basic diagnostic information about troubleshooting VDA registration issues. The download link details can be found in Chapter 2, Troubleshooting Toolkit for Citrix XenDesktop®.