Smaller predators such as weasels can enter through chicken wire, and squeeze through amazing small cracks. You might try having someone shine a flashlight around in the coop after dark, from outside you may be able to identify gaps in your defenses. Also, hardware cloth is best at the bottom of fences (and covering windows and doors) - not sure how small the openings at the bottom of your fence?
If the weather is hot and your chickens are still sleeping bunched together for comfort, you may have them smothering - as said above, place them on the perch after dark to encourage them to start roosting. Good luck.