I've had problems with that particular mission before, even got 6 friends to come in to the mission map and have a look around for the missing Arachnoids. We found a couple, but then /petitioned and waited for a GM. Turned out some of the critters had spawned outside of the map.
I'd suggest getting a few friends to come in and scout around, just to make sure they're not hiding somewhere silly like behind the entrance (the cave goes back a little way behind it), and /petition for a GM to pop along and see if they can find them for you.
Help Dirk Knightly the freelance detective solve a case in Arc ID:368097
The 2nd mission from Terrence Dobbs in the "Biff" arc ("Clear out Arachnoid tunnels") seems to be broken. It's a kill-all, and I've played it through from scratch twice, clearing the map each time, but it won't end.
According to ParagonWiki, 3 waves of ambushes are supposed to come when you're fighting the boss, but every time I play it I get only 2 waves, so maybe that's where the problem lies.
Not sure if this started with i21, but I've never had troubles with it before.