Have you tried golf balls in the nest box? That sometimes helps. Sometimes not. I have 2 juveniles that insist on laying on the floor of the coop. Drives me nuts, as whenever I see them start to go into "egg lay mode" I pick them up & put them in the nest boxes...they jump right back out. Sigh. I dunno if they'll ever stop laying on the floor.
I have one that's been laying a few months longer, and she laid on the floor every time she laid - right by the pophole door, in fact. She's in a different pen, even. Well, she took a 3-week break from laying, and started back up last week - now, she's all of a sudden laying in the nest box! So, with some, it just takes a bit of time. Good luck!