I think meowracer makes a good point. If you re using 3/5/8/5/3 on the shoulder exercises, this is just fine!
The thing to remember about PUB is that it is a very fast paced endurance workout, not a strength workout, although you will make strength gains if you are new to the workout. To lift heavier, you need to have longer rests in between sets and lower reps, as Kathryn suggests. I can lift much heavier weights on S+H than I can on PUB. Not a surprise. This is why I often do my own thing, so I can take the rest time I need to ensure I continue to make progress with weights.
I find that drop sets also help to build up the poundage. This, you will have to do on your own because Cathe has no DVDs that follow this method. You target each muscle group with 2 or 3 exercises. You might start bench press with 50 pounds on the barbell and only be able to crank out 6 reps. Great! You immediately drop the bar to 45 pounds and crank out maybe 8-10 reps. Immediately drop again to 40 and crank out maybe 12-15 reps. This is what I do. I continue all the way down to 25 pounds, decreasing the weight, amping up the number of reps but not usually going beyond 20 reps, not much gains from that. Once I hit 25 pounds, I switch to endurance mode and crank out faster single reps, interspersed with half reps/low ends as Cathe uses on many of her endurance DVDs like CTX upper. remember though, that with drop sets, there are no real sets and therefore no real rests between pound loads. You only rest for the time it takes to take plates off the bar, or pick up the lower size of dumbells. Note also that platemates are very useful for gradually increasing your loads.
Each time that I do this type of workout for the upper body, which, as an ectomorph, is where I have trouble increasing the poundage, I can gradually increase the number of reps I can manage at 50 pounds. Once I can hit 10-12 reps at that weight,I'll increase my starter weight for drop sets with bench press for chest.
I do this for each upper body muscle group, spread out over 2 different days of the week, once every 2 weeks. It exhausts each muscle group and helps me make strength gains.
Clare