I was just testing this out yesterday on the ground. I did notice that the Dual XGPS was slow to acquire a GPS signal while set to ADSB mode. Once it did acquire a GPS position (it took a few minutes, and up to 10 minutes on one test run), iFly picked up the Dual's GPS position immediately. I attributed the slow GPS acquisition time to being on-the-ground.
On-the-ground, the GPS mode for the Dual works much more reliably than ADSB mode.
From an iFly perspective, to determine if these is any issue with connectivity between the iFly and the Dual XGPS, select "Menu => About => Connected Devices", and after selecting wireless device "Dual XGPS", make sure the bottom status box shows that we're receiving "Heartbeats" (bottom-most number). If we're receiving heart beats, then from an iFly perspective, everything is connected fine.
It sounds like your issue is with the Dual XGPS not acquiring a GPS signal while in ADSB mode, correct?