Sometimes you fish a run or pool and catch a few but just can't seem to pull all the fish out of it that you know are there. The following tactics could help you to mop up after your standard methods have been tried.
Move your flies - After several attempts at standard dead drifts try lifting, twitching, or stripping your nymphs or dry flies. This will often entice an aggressive fish.
Dredge the bottom - A couple of really heavy nymphs like Tung Teasers or Wired Prince Nymphs fished Polish or Czech style will bump along the bottom and get down deep where fish like to hold when they aren't particularly hungry.
Pull a streamer - Occasionally fish will attack a streamer even when they have refused dries and nymphs. Simply tie on a streamer like a Slumpbuster or Zonker and work it through the likely spots with erratic strips.
Try one or two of these tactics the next time you're sure that a run just has to be holding another fish or two.