Scott,
If the box is 12" off the floor and even if you had a ramp the chicks probably couldn't get back into the box. They just don't understand. I would put the nest box on the floor or take it out altogether. She will snuggle with them on the floor of the coop and they'll be able to get to the food and water which they will need by now.
Hear, hear to this! I put a small platform in front of my nestbox so March could get the chicks out. Once they were out, she brooded them just fine on the floor of the coop. It was sub zero weather. But I had a 65 watt light in the 4x6 coop to keep it above freezing.
One thing I would do is remove the cock from the coop. Also not have the shavings too deep. I had problems with both. I forgot the hen was wary of her cock. Even tho he didn't hurt anyone. She dug a real deep hole in the shavings and settled the chicks inside to protect them. In the process, one chick got stomped and died. It was an accident. Without the cock in there, she didn't dig so deep. They are 5 weeks today and feathered out. Starting Sat. it will be warm enough to put them out. March is really looking forward to that!
Best,
Karen
I've had some figure out the ramp just fine and others choose not to use it and brood on the floor. I've also had my best results with the cock bird left in the mix.