It can be number of things. Let me give a few to check.
1. Setup - Design Rules - Nets. Select GND net (or the net name in question), Routing icon, under 'Toplogy Type' (upper left corner) select Minimized. After that go to Tools - Length Minimization (or Ctrl + M).
2. View - Nets. Select GND net (or other) in right pane View List, under it in View Details select 'All Except Connected Plane Nets'
I tried both of your ideas, but no luck. The Length Minimization usuallly works, but not on this board. Any other ideas?
Check In Tools > Options > Split/Mixed Plane to see if Update Unroute Visibility is checked.
OK, cool, that worked. Thanks Janet!