Just spent a very frustrating day on a local fishery. Caught one fish......but watched dozens feeding readily on something just under the surface, but couldn't make out what it was.

Tried drys, wets, buzzers, lures, all to no avail. Is there anything worse than catching next to nothing when all you can hear is the sound of fishes' mouths slapping as they gorge themeselves!
Any ideas as to what I can try next time? Any help greatly appreciated.
PS. Water is only 6/8 feet deep and very rich in natural food.