That's a tough one and we were talking about similar behavior not too long ago. Trout sometimes in low clear water will feed on insects that are not even on the water. You might want to fish for those fish by working a top dropper on a wet fly rig. Partridge and herl should work to imitate the black caddis. In Davy's video he shows how to do this. If you have it give it a view before heading out.