Are you using Nugget Doom? There's a setting you'll need to change for that to happen, I think it's in the Weapons menu section.
Unfortunately with the way Doom's gibbing code works, you won't see a lot of the higher tier monster gibs without Nugget Doom's force gib feature (or by setting the bfg ball damage to around 10,000)
It's a real shame Doom's SSG, zerk fist and chainsaw don't gib by default though, few things are as frustrating as slapping an imp while berserked and watching him limply fall over instead of exploding into a pile of ex-imp lol