It's normal... at 85% you should be able to do 4-5 reps in a set. But the goal of the program is NOT to fatigue yourself.
Do single reps, do not group the reps. You can take short rest, 30-45 seconds between sets, but do singles. FOCUS ON QUALITY, SPEED, DOMINATING THE WEIGHT ... contract the muscles as hard as possible. Force your body to work as hard as possible even if the weight is easily done.