HttpClient not reaching url

I'm writing a WP8.1 app that reads and sends data through REST API. All of them work without problems excepts the "search" one.

This API works in POST and I need to send a string in Json format defining my query. If I set only few filters, the REST API takes too long to respond. Therefore the HttpResponseMessage object has no success code and it returns a "NotFound" message, even if the web API is correcly runnning.

If I try the same request via Postman it works correctly, but if I try it via SoapUi I get a "java.net.SocketTimeoutException: Read timed out" error message.

In SoapUi I managed to extend timeout in order to get a response, but my C# code keeps on not working even if I set the TimeOut property to HttpClient object.

Can someone please help me to resolve my problem? Thanks!