when you get stuck at a weight, you repeat that weight the following week. if you've been progressing for a few weeks, chances are you'll hit that weight the following week (my BB curl stalled 3x, but the following week i'd hit all sets/reps). if, however, you don't get all 5x5 the second week at the same weight, then the following week, bump the weight up and do 5x3. do this for about 4 weeks, increasing the weight every week. after about 4 weeks, drop back to the weight you were stuck on with 5x5, and start 5x5 with that weight all over again until you get stuck again.
as i understand it, the principle behind this program is that you're constantly varying the weight, which is what your muscles need in order to grow. even if you go to 5x3 for 4 weeks, then drop 20lbs and start 5x5 all over again, you're still creating a positive change. even though the weight is lighter, you're now doing more reps. it is because of this constant variation of weights that you want to start the routine with a light enough weight for each exercise. focus on form and not going to failure. once you've been progressing on this routine for a while, you'll get to the last rep of the last set and just barely be able to finish off, only to come back in the next week, bump the weight up, and repeat. i do not want to hit failure on any set, because that means i can't go heavier the next week. being able to continiously go heavier week after week is a real boost.