This is a pretty common problem with ramps. What often happens is that the hen goes to the top of the ramp and tells the chicks to come up. She may hop or fly up there, not take the ramp herself. The chicks don't know to go to the base of the ramp and walk up so they can't get up there. It's not that they don't want to, they just don't know how.
Yeah, I'd want them locked away from predators. And you don't want them to get used to sleeping out there. I don't know of a good way to train them to use the ramp. Food might work but your other chickens would probably destroy any plans with food. I don't know what your coop looks like either, how much room you have or how it is laid out. When she gets them in the coop, expect her to take them to bed on the coop floor. She doesn't need a nest, just room on the coop floor.
I see two options for you. As others suggested, put them in each night until they learn to go in on their own. That way the broody will at least want to get then inside since that's where she's used to them sleeping.
Another option is to build a predator safe pen at ground level and train her to take them in there at night so you can lock them in. I do something like this, not because of ramps or that they can't get back inside but because my main coop may be pretty crowded as they get older and she weans them. So I have a separate place they can sleep at night. Integration is not a problem since they roam with the rest of the flock during the day. When I thin out the flock sleeping in the main coop to make room they can move in. If there is room in the main coop I let the broody sleep with them on the main coop floor.
I don't know what your goals are, how you are set up, how crowded it is, anything like that. I'll just present a second option if you see any benefits with that. But I will issue a warning, be careful if you pick up a broody hen with small chicks. Chicks sometimes like to crawl up under a hen's wings or feathers. I once killed a chick by picking up a broody hen, not a good feeling. I crushed a chick that was under her wing. I still pick a broody up if I need to but I am very careful when I do.
