When tying nymph patterns most of us add a little weight to the fly by wrapping lead wire (or some other heavy wire) around the hook before any fur or feathers get tied on. Of course this isn't necessary to do & you should tie some nymphs without any weight. That aside, how we put that weight on the hook, with the wire, does have an effect on how your fly will perform in the stream.
What that studious group of anglers discovered was that nymph patterns that were of a flattened shape tended to drift through the water much more like the natural. That was the key to a more natural drift.
So, how do you get your nymph patterns more flattened? Easy, just flatten that wire on the hook before you put anything else on the hook. So when you're tying a nymph at the vise, wrap your weight around the hook, then take a pair of forceps & flatten the wire - crush it on the top & bottom & push it out at the sides. Then go about tying your fly as you normally would. When you bring your dubbing around the flattened wire it will follow the shape of that wire & have a more flattened appearance. You might have to flatten it a little more when you're done tying it, depending on the amount of dubbing you use, but usually not. Flattening the wire weight is usually enough.
Most of us add our weighted wire to our nymphs in the top third or half of our flies. That's fine. Look at a natural nymph & compare them to your fly. Getting that thorax flat seems to be the most important part.
You can add different amounts of weight to your nymphs as you tie. In fact, it's a good idea to do so, that way you'll have some that are heavier than others for getting deeper in the stream. If you do, I suggest that you color-code the head of the different weighted nymphs with different color thread. This way one glance will tell you if you have a heavy, medium, or light weighted nymph.
It turns out that if you don't flatten your nymph & leave it round, than it will want to float or drift upside down, with the hook bend pointed towards the surface. Real nymphs don't do that. They tend to rock, bounce, & crawl along the bottom with their bellies & legs pointed down at the stream bed.
So flatten your nymphs. You went to all that trouble to tie those flies with all that detail. What a shame it would be if you were missing fish because it was drifting upside down!!