Most skirmish maps have the nexi on plateaus.  In many cases every possible build square gets used.  If you have neither radar nor vision on the plateau it's very hard to get a juggernaut to go up there.

It appears that the juggernaut is refusing to move to a square that's occupied by something you can't sense.  It just sits there, taking fire if the defenders have anything that can do ballistic fire.  If there isn't orbital protection you can do a scan and then it is quite willing to invade.  Likewise, if you can get an aircraft to the scene you can then invade.  If you have radar coverage it can attack anyway.  (Note: I suspect radar doens't help if you are using something that only has direct fire weapons.  I haven't found myself in the situation, though.)