sharing nest box not happening

It sounds like you have two hens of different pecking order status, and when the higher ranking hen is in the box, the lower ranking hen doesn't dare encroach, even though it's not uncommon for two, three, or four hens to crowd into the box at the same time to lay. I've even witnessed one hen straddling another to lay her egg on the back of the hen in the box.

Most of the time, hens are able to hold the egg back until they can find a suitable place to lay it, but not always. It will just slip out. I've seen that happen to hens who were forced to hold back, and sometime during the night, the egg slips out and breaks on the poop board under the perch.

Adding a second nest box isn't any guarantee that it will solve your problem, but there isn't really anything else you can do.

I have nineteen hens and six nest boxes, but they all insist on using one favorite box. And even when one is occupied, if its by a hen of a higher rank, the hen-in-waiting sometimes is reluctant to use the box next to this hen, standing on the perch waiting her her turn in the favorite box.

Sometimes you just aren't going to be able to get chickens to do what you want.

 

I would just put another nesting box in for her to use.
Maybe just as a temporary solution.
I used a 5 gal bucket, they love it and I didn't have to buy or build anything.

Then when they get older, if you find they are just using the one box you could always take the other one out.

 

I have 3 hens that are laying(1 BO and 2 SLW) and 3 nest boxes. The BO seems to be at the top of the pecking order. If she is in the nest box, the other 2 will go to another but if she is not then they will both go into the favorite nest box together. I just used the wooden fruit crates. I tuned them on their side, put the open "top" against the wall and knocked out the bottom slats except for the top one and the bottom one (to help keep the bedding in)