This time of year is murder on new layers and their first eggs. Their bodies may be ready, but the short days just aren't producing enough daylight to trigger laying hormones. It's a waiting game, and that's about all you can do. Luckily, days are steadily getting longer, and that mere fact sometimes is enough to get those stubborn hormones moving.
Even though her hatch mate started laying, this one may have slightly different genetics. Also, if a pullet manages to begin to lay before winter, they usually keep on laying through the winter. It's an individual thing.
Meanwhile, make sure her diet is balanced with a good commercial all flock feed with enough protein to help her finish growing. Save the layer feed for after she starts to lay since layer is not adequate for growing chickens.