If the SkyRadar-L sends any traffic data to the iFly, it will be displayed. If you are getting weather and no traffic, it's pretty clear evidence that the data just isn't there - it's not a connectivity issue.
As many have said before, traffic will be very iffy with a 978 receiver. For full traffic you need an ADSB Out device. I advise customers that "Receivers are for Weather, Transceivers are for weather and traffic".
-Walter