I'm not Robert, but I'm going to chip in anyway. Hope you don't mind.

I fish nymphs ranging from no extra weight all the way up to lead wraps all over the shank and bead chain, and will sometimes add a piece of shot above them for even more weight. Even a lightly weight nymph will get to the bottom eventually, if you have the time to let it get there, and sometimes the fish like to see something sinking slowly. Other times, the fish are moving fast and in a couple feet of water. If your fly can't hit the bottom like a brick the fish will be gone before your fly even gets close enough for a look. Also bear in mind that heavy flies hit the water differently than unweighted flies, and sometimes all it takes is the sight or sound of a little bug dropping to the bottom like a rock right in front of a fish's nose to spook it into the next county.