I am still new to coops, but one of my customers told me that if the coops is a little small the chickens may be crowded enough to want to stay outside. Another reason could be is that it could be too hot in the coop if it is not ventilated enough. Last think I can think of is if the food is outside of the coop maybe they wish to stay next to their food more often. I could be wrong about this, but I hope this helps
They might not like the ramp for some reason. Try putting them in yourself every night and locking them in, eventually they should figure out that they need to go inside. I agree also to try putting the food inside.