Hi Vangelis,
does dotnet send the message using a default namespace e.g. "http://tempuri.org"?
If so, please define such a namespace at the B1 inbound definition.
The following screenshot is taken from sap.B1Mobile standard scenario, it's the same principle for http inbound:
Please also check for the correct identifier of the incoming message and reactivate the scenario package after changes in inbound configuration.
Best regards
Bastian
P.S.: Does the inbound step cause an exception? You can check it in Monitoring -> Process Monitor.