WCF multiple async. call in WinForm

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C#

WCF

WinForms

asynchronous

I'm trying to interface between host and client with WCF.
I made two type of the host.
One is the host based on WinForm. other is the host based on Console.
All the rest of it and are the same.

In console case, when a client calls async methods mutiply, methods are called normally.
In winform case, after async. callback, next method is called.

I'd like to call async. method multiple in winform.
Could anyone let me know the reason and solution?

if circumstances require, I can send source code.