When stripers are keyed in on live bait, it can be hard to turn them on anything artificial. The trick is to give your fly a different sound and vibration than the live bait. Cast to the outside of the bait school and work your popper erratically, constantly changing the number of pops and the length of pauses, until you hook up. Then repeat. Still not connecting? Switch to a foam crease fly, which pops and dives a few inches on the strip.