I have had chickens for many years. I started out with a old garage for my coop. Boards were loose. Gaps and cracks were everywhere. Things could dig there way in. Not good and many of my chickens paid the ultimate price for my lack of knowlege.
I finaly got tired of loosing and replacing my birds and of patching up the stupid garage, tore it down along with 2 other decrepid outbuildings and my husband and i build a new coop from the old lumber we salvaged.
My chickens are now safe at night while roosting.
You must walk around your coop and look for and repair all the points of entry. Hardware cloth is a chicken owners best defense. Chickens are absouletly helpless at night, and they depend on you to keep them safe.
If a fox is getting in, thats a pretty big hole or opening.
I recomend also a game camera to point at your coop. Another tool a long time chicken owner is giong to need. They are not as expensive as they used to be.
I am sorry for the trouble you are having!