As you're training them to go to bed up the ramp on their own, just leave them for an extra 30-45 mins later than you have been doing. Unless they feel a sense of urgency to get inside when it gets dark enough, they won't go in of their own accord. If you do find them huddled at the bottom of the ramp, you can then put them in, but you do need to give them a chance to work it out for themselves, now they've lived with you for a few days.