WCF Workflow service application and state machine in WF 4.0

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Silverlight

WF

.NET4

Hi
I am working a project where I need to create a state machine work flow in 4.0. I have installed .net framework 4 platform updates and I am able to get state machine and states in toolbox to work with.
I have created a WCF workflow service application. I have currently 3 states in this.

State1 which executes a external process.
State2 which updates a database.
State3 populates some entity class.

Each state will be having a receiveandsend activity. I need to trigger each states individually from client. Cancreateinstance of each receive activity is made true.

I have to call this from a silverlight application. So i have added the service referance to this workflow in silverlight application.

When I execute the first state, its works fine and external process is executed.

But when I try to execute the next state by calling the next receive activity from client, its showing error as below.

"Operation 'ExecuteExternal|{http://tempuri.org/}IService' on service instance with identifier '4732d197-32c0-4591-87bc-fa0adb7ec43a' cannot be performed at this time. Please ensure that the operations are performed in the correct order and that the binding in use provides ordered delivery guarantees."

I think the WF is not waiting after first state is finished. Can anyone help me to solve this?