I struggled with this issue for a long time with both a Garmin GTN650 and GTN650Xi. I finally discovered that the GPS antenna (non-Garmin, small 1" square) on the G5 was interfering with the WAAS GPS antenna for the GTN. The G5 was on the dash, and the WAAS was behind the slider canopy (5'ish?). I replaced the G5 antenna (non-Garmin) with a Garmin antenna and problem was solved. Lesson learned was that the WAAS antenna is susceptible to interference. It was surprising.
Place the aircraft outside with plenty of sky. Boot up your GTN and go the the systems display that shows the satellite signals. Make sure the canopy skirt is not covering the WAAS antenna. Then, pull the CB or turn off everything else. EVERYTHING! If you get a 3D location quickly (less than 2 minutes), it's a good indication you have another component interfering with the WAAS. Turn on your electronics one at a time and closely watch your satellite signals. When you see signal degradation, you have found your problem.
That all said, I also had a problem with a unit and Garmin had to replace it under warranty. I discovered this by taking it to an avionics shop and having them perform a bench check.
$