I had a similar experience with my chickens. The first night I brought them in the coop, none of them could figure out how to get back up the ramp. They could get down just fine. By the second night, the oldest chicken I have figured out that she had to go up the ramp into the coop. None of the others could figure it out though. By the third night, some of my chicks were experimenting with the ramp, but still would not go up. They would huddle in the corner and try to sleep there.
By the fourth night, the chickens who did know how to go up eventually taught the others to as well