Well the simple answer is to hit all your reps. You shouldn't really make any contingency plans because that is the goal of the routine. Realistically, there's going to come a point where you aren't going to be able to hit all your reps.
I would say, if you miss even one rep, do the same weight next workout. If you're not able to hit all your reps the next workout (or at the very least improve the number of reps you hit) take a look at your diet and drop 20lbs from the lift and work your way back up. If, after the deload, you still can't move passed the weight you were stuck at, it's time to try something other than SS (assuming your diet is in order).