It takes quite a while for new members to truly become part of the flock most of the time. That is very likely part of her issue. Even though the Isa's are friendly to you, they may not be as nice to her. But even when they don't go out of their way to be brutal the pecking order still usually stands pretty strong. She probably is the lowest being a new addition.
By quite a while I mean about 4 months or longer. And sometimes they integrate more cohesively when there is a good mix up (departing) of the original flock. In other words breaking up the cliques they seem to form new ones. Though I'm not suspecting that's your plan or even suggesting it, but just sharing.
My Barred Rock have been a mixed bag of personality. One really calm and hangs out on my lap as long as it's on HER terms. Others pretty shy like you say. Still one of my favorite breeds and after raising more than 100 birds of MANY different and even rare breeds, I have 1 girl BR girl who has made the cut to pet status many times.
Hopefully she will settle in before too long.
Alternatively, you could set her up a lay box where she currently lays and get a reachy grabby tool mentioned by WVdc. Though I understand that isn't truly ideal.
I guess to be honest... if I have a girl who consistently refuses to lay in the boxes... she finds a new home. I have 1 girl who refuses to lay with the other hens most of the time, she also made pet status because of her personality. I hate searching blackberry bushes for hidden nest. (it's always the EE!) At least now she is laying in my Silkie coop. The worst part is that very often chickens follow the "monkey see, monkey do" routine and happily teach each other bad habits!

Gotta stop fence hoppers, nest hiders, and egg eaters.

So ya, now I have 1 Silkie who lays in that (about 2 bird) coop (and belongs there), in addition now an EE and a production red. Oh and most recently one of my broken broody's Silkies from another coop started laying there. Mind you there are only 2 boxes and they all lay in the same 1. There are 10 boxes available in my big coop where the 3 USED to lay... and only 13 girls, 4 of which haven't started laying yet. Crazy birds!