This is something I have never been able to do with web services. I tried every trick I could think of! I even tried to write the SOAP request manually including the name of the command from BCS to no avail. I have heard some people may have had success adding an item but never updating one. I had to use the javascript client-side object model code (CSOM) to do this and this works fine. I still use SPServices for most everything else. There are plenty of examples on doing this in JSOM/CSOM out there.
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