You don't only add 5/10lbs a month, you add reps as well. Remember, it's a 5+, 3+, and 1+ set. That means those are the minimum numbers, and you do as many as you can.
The first time I ran the program, I was adding reps along with weight every cycle because the program was making me stronger each cycle. It should have the same impact for you. If it's not, you're most likely undereating, or fucking around too much with the assistance work.
Truth be told, most people fuck up westside type programs because they only train their strengths/things they like and avoid their weaknesses. That, or they can't figure out how to rotate maximal effort/assistant work in general. I would say that, if you have these sorts of questions at this point, WS4SB would be a poor choice. 5/3/1 is very simple and laid out already, making it a good choice for this situation.