Getting: The request failed with HTTP status 400: Bad Request. on 1 method. Works on the rest.

I have a Web Service located on my local box.  It has 6 methods in it. 

The client application I'm running hits 2 of those methods perfectly as I can break into them when attaching to my w3 process.
However, the one method which I need never gets hit. The client gives me the message:

System.Net.WebException: The request failed with HTTP status 400: Bad Request.
   at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
The method that fails gets sent about 120 KB of XML Data. The other 2 get only a tiny bit ...