Vary the length up if nothing is happening.
Or, if in turbulent water, e.g., tide rip
I have had success with as short as 24" at times, usually in turbulent water.
My theory is that salmon seldom travel alone and are opportunistic. If one fish slashes through some bait fish, and there is some spillage of wounded herring, another salmon in the same area may swoop in and chomp.
Given that the flasher/dodger imitates a salmon, and the lure imitates the wounded herring ...
But I could be wrong.