Those s̶q̶u̶a̶r̶e̶s rectangles are exactly what the navigation mesh looks like, which means (as long as the mesh is covering the skeleton spawn), the problem lies elsewhere.
When you're testing the skeletons, is there at least 1 player (or bot) in the map in a place that the skeletons can reach (so not in their spawn)? Skeletons won't move unless they have a target that they can reach. If there are no players in the map for them to attack, they won't move.
If the skeletons still won't move, try getting within melee range of them. Do they try to attack you, or do they stand idle?