You want to try and imitate helpless creatures or one that's relaxed and moving slightly. Varying the retrieve can help. Rapid fly-line strips of a few inches long up to a couple of feet in length imitates a panicked bait fish such as a minnow. . Big fish can be provoked to attack large big bass bugs that intrude on their territory. Try retrieving a bass bug past some water-lilies where these predators lurk.
Large flies can also draw a strike from migrating salmon for the salmon reason.
These salmon have changed to a bright color on the migration back to their spawning areas. They do not feed much but defend the territory they have staked out for themselves. The small dark flies that you used with such success in the summer will not work. You need a big colorful fly to provoke an aggressive strike reflex.