At 3.0-3.5, the main frustration I have with S&V (or just coming to the net in general) is when the opponent constantly lobs you successfully. Then, you look like a moron repeatedly. It is also VERY annoying to spend all that energy getting to the net only to sprint back chasing some annoying lob.
The point is, you really need to choose your opportunities well. This is what I am trying to develop. Go in when you know its a deep ball to their backhand. They aren't going to lob you there. Easy shot to their forehand? Don't go in to the net, idiot. They will lob you.
I also sometimes wait too long. You need to be charging the net as soon as you hit the ball, and can't wait for visual confirmation. ie: By the time you see that it's a deep shot that's good to go in on, it's too late to go in. This needs to be a 6th sense, knowing where the ball will go the second you hit it.