SmartApp not parsing xml file during device descovery

I’ve written a windows app, emulating a smartthings type device
Using the Generic UPnP Service Manager SmartApp, I’ve gotten as far as Smartthings requesting the xml file.
The Generic UPnP Service Manager SmartApp finds all the devices as requested by their urn. then goes through the verification process using the ssdpPath to request the xml file.
It pulls the xml from all of the devices except the windows app.
The http server on the app shows the file being requested and accessed by the hub
The file is accessible and correct as far as I can tell, it can be read from a web browser using the correct port number.
Smartthings logging displays the hubaction request and the callback, but for my app the callback never gets used.
Anybody got any ideas as to what I should check next ?