Spiders seem to almost always have some quirks to them. Sometimes you can't hit them. In other missions they don't collide with some geometry making them walk sunk into the floor (and also harder to hit). Their ragdolls most of the time go nuts if trying to drag them... Spiders have always been pretty buggy. By the way: Open the debug console and enter noclip followed by the return key to activate the fly mode wich is perfect for getting yourself unstuck. Repeat for disabling the fly mode