Sorry for your losses, they are frustrating, especially if you don't know what is doing the killing.
Are your chickens being killed while in the coop? The run? Or out in the yard? A secure setup should rely on multiple levels of defense. I have my chickens in a 1/2 acre yard that is surrounded by 4' high electric net fence. This is the strongest deterrent, and keeps most land predators at bay. Inside the yard are a very large apple and pear tree, so 70% of the area is underneath their branches, and this helps protect them from flying predators. I don't let them out into the yard all day, every day, but mix it up based on weather and my schedule. Most of the time they are out in the evenings, but on weekends it could be the mornings or afternoons. Mixing it up will make sure there is not a food supply outside every time a predator comes by. Then I have a 15x15 covered run. The roof is aluminum panels, and the sides are 2"x4" field fence on the outside of the 2x4 construction to keep out large predators, and 1/2" hardware wire fencing on the inside to keep out small predators. Then the coop is inside a 12x16 shed which has plywood outer and inner walls, and double pop doors - one on the outside and another one 3" later on the inside. Finally, I have a good rooster. He is the active warning system, and I have seen him sound the alert right before a predator comes or flies by. When I first started raising chickens and ducks, I was losing 1-2 birds a month. For the past 3 years, after re-working my coop, run and yard many times, and adding all of the above, I have not had a loss to predators. They are still out there. I have 3 trail cameras within 200 feet of my chickens and ducks, and I see them all the time - in the past 30 days I have pictures of bear, bobcat, coyotes, raccoons and oppossums, but they quickly decide there are easier places to hunt.
If you want to do 1 thing to protect your flock, I would go with the electric net fence around the yard area. That made the biggest difference in protecting my flock. Good luck!
