Server did not recognize the value of HTTP Header SOAPAction:

I got the error when I called a webservice
---- Server did not recognize the value of HTTP Header SOAPAction: http://tempuri.org/GetBestBidOfferData. 
The odd thing is that I have several methods in this webservice, and only two of them got me the error message.  It used to work, but then gave me the error message after i added some other methods to the asmx file.
The namespace should be  the same for all the methods. One more thing to note is: these two methods had return data type DataTable before, and I changed it to DataSet. It should not affect anything, I assume.
I reaseached  on web and still could not figure out the problem.. please help
Thanks
 
 
 
 
 
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