You can lock them all in for several days to a week until she starts laying where everybody else does. Once she starts she should continue. But you may have to retrain them on occasion.I'll have a look around where she usually goes! If I find she has been laying somewhere, what can I do to make sure she lays in the nesting box in future? And she is indeed a Light Sussex!
My birds often look ready to lay at least 1 month before they actually do. Another indicator I see is them walking around making a "squawk awk awk" type sound. Sometimes getting really loud just before laying.
Thanks for letting me guess on breed!