Check your polar diagram, it will still show what you could once get.
Save it, file, screen shot, save to file.
Then go into preffs and clear the file. It will then start replotting it. Let it run for 24 hours or so, and see if it is vastly different to our original one. (see mine above)
If nothing else in your rig, or your environment has changed contact AirNav support - you could have the same problem I have (still unknown, but very real)
9M