Alberto,
For clarity, am I correct to understand that when viewing the Skyradar stats under the Connected Devices dialog, you see "0" for # of reported metars, tafs, etc? If so, this indicates there is a connection, the iFly is receiving heart-beat data from the SkyRadar, but there isn't any weather data being sent. Which means the SkyRadar is not receiving any data. Where are you flying, and what altitude? Are you confident you are in a covered area? And are you sure the ADS-B antenna is properly installed? (You are correct that the GPS antenna isn't needed). If answers are all Yes, I recommend you call Radenna...sounds like your SkyRadar might be shooting blanks.
Thanks,
Walter