Yes, the 5 lbs is probably too light to register on the digital scale. You could try a heavier weight (if you have one) or, like what was already suggested, get on it yourself with the weight in your hands and then put it down. See if the difference is 5 lbs (and then do it on you old scale, too).

Also, make sure both scales are on hard, level surfaces. It doesn't take much to throw them off a bit!