I had a RIR that absolutely just chilled out in the egg box for 2-3 hours sometimes. She was done laying, she was just hanging out (I think she was just taking a break from her sister who was kind of intense).
With my new girls, we had a lot of consternation because my laying hen always uses the far left box, so when the three POL pullets started laying, they also wanted to use that box (it was also just a touch bigger because the hen is a big girl). One tried to get in on top of her - glad I was there for that.
We widened the other boxes to make everything equal, and put the younger girls in the other boxes a few times when they were nosing around in the coop. We just kept putting them in the other egg boxes, and eventually one of them had to lay so bad, she accepted the new spot. Now the pullets use all three boxes interchangeably, though there is still a preference for the first box.
All of which is to say, if you've got a parker (rather than a broody), you're best off teaching the other girls that the other egg boxes exist and are cool. You're likely not going to change a bird that takes a long time to lay or who enjoys her alone time.
(Though double check her eggshells. Sometimes a long time to lay means they're struggling with the contractions; calcium can help.)