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.