Nymphing a large pool today, I noticed some rainbows along the very bottom towards the end of the hole. They weren't moving much at all and my fly wasn't getting deep enough quick enough, so I threw on some split shot, rolled my line up to the head of the pool, let my rig sink, and started tight-lining. On the second roll, a large rainbow, close to 18", sipped my prince and took me for a ride.
Don't be afraid to add weight in order to get low, especially as the sun sits lower on the horizon and the days stay colder longer. I've noticed that trout seem to stay closer to the bottom in these cold months.
Other than simple time on the water, the biggest tweak to my nymphing game I've made this year that has yielded a ton of trout is adding weight.
For more on it, I highly recommend On the Trout Stream by Joe Humphrey.
"You must constantly adjust your weights and weighted nymphs...to get your nymph down on the bottom..." - Joe HUMPHREY