3-4 DE arn't supposed to be big pass rushers. Remember, in a 3-4 defense your defensive line men are the cogs that prevent running lanes. They are the meat that prevents push at the line of scrimage. If you have those guys twisting or trying to speed rush around the end, who is going to clog the hole that they leave? These guys can pretty much only bull rush because anything else will expose the linebackers to huge offensive linemen. That my friend is a no-no. These guys can stunt, but they must be able to contain their gaps if they do. Thats gap(s) as in two. The 3-4 is a two gap defense unlike the 4-3 each player is responsible for a single gap. This means it's not uncommon for them to take on two offensive linemen. I don't mean them get double teamed, I mean them getting in the way (clogging the lane) of two offensive linemen. They do not stunt much that I've seen and I'm sure there is a reason for it. Probably because they have trouble containing their gaps.
Of course that doesn't mean they shouldn't be applying any pressure on the QB. It just means they arn't expected to be dominating pass rushers