Whenever I spawn skeletons on my map, they all cluster at their spawn and can't move. I was thinking this might be because while I'm in a different part of the map triggering the spawner, they have no enemies to run towards. However, smaller skeletons that jump away from the main spawner still won't attack me even though they aren't stuck.
The skeletons need a navigation mesh to be able to do anything. You can make a quick basic one with nav_generate.
Use the nav_generate command. This generates the walking space in the map for bots and monsters to take so that they can move around. Also, have sv_cheats activated when you generate the .nav.