What was the depth of the water in which the fish were lying? And what was its temperature?
If the water is very cool, it's often the case that the distance over which the salmon will commit to a fly is reduced from the normal 6 feet or so. That distance is also affected by the current beyond the bounds of the lie: they're often reluctant to follow a fly into heavier water owing to the energy expenditure involved.
And if the temperature is normal - in the range 7.5 - 15C - but the fly is perhaps a mite too far away, they may move to inspect it but not commit.
For every one reason for a salmon to take there are 99 for it not to take.
A bit to rational, and technical for a wild species. Personally it can be the opposite of norm as many times as norm thinking with finicky fish.
On trick I would certainly off tied, would been full floater, 16ft tao
pered leader, and a small heavy cone head Francis Dead drifted down to them, even letting the line slip thew your fingers, bopping along the Botton, till the francis if swirling round the fish lie, Your spotter can telll you. Lift and drop the rod. If it works , bang your in. if not the fish will move with lockjaw.
A lot resort, that works when all elz fails, And it different.
But remember it Salmon fishing, and some just dont, play.