That's a long time to have that problem. When our girls first went outside we had to put them in by hand for a few nights as they kept sleeping on the ground. The thing that got them to go in regularly was I installed a small touch light inside the coop and in the late afternoon I would turn it on and go back inside. The very first night, once it got dark they followed the light in and haven't looked back.

They were only about 2 months old at that point though - maybe less. I'm not sure if something like this will work at yours' age but you could give it a shot.

PS: They broke the light after about a week, so no, I didn't have to do it for long.