1) WCF Service must be having 'basicHttpBinding' configuration
2) number of parameters, their names, order must be followed as it is.
3) Do not press the 'Refresh' button
4) Write the Query command correctly
5) Enable the following web.config setting in service. This will allow you to view the actual exceptions raised.
even a small mistake will make your life miserable. So, Please take extra care.
Enjoy!! :)