What did the rise forms look like? Was there an obvious bubble and slurp like they were taking a bug off the surface? Or was it more of a splashy, porpoising situation (seeing more dorsal and tail) consistent with emergers being eaten in the film.
Any bugs on the water at the time? Could be that your pattern was too big, not sufficiently surface flush (if they were taking cripples, spinners, or just picky overall due to pressure), or maybe your casting and presentation still need a bit of tweaking.
In slow water, the fish have an eternity to inspect your offering so you’ll get refusals from things like micro drag, not going fly first downstream with a reach cast, leaders that are too short or too thick (this time of year is 5-6x season for me and I like em long, also prefer using fluorocarbon bc of its refractive index and tendency to not stick right in the film).
If it was an emerger bite, then you could try a dry dropper (short tag with a lightweight hares ear or PT can do the job) or present lightweight nymphs (no tungsten) and wets on the swing next time.