Check she's not egg bound (doesn't sound like it, but best to be sure) up their grit/oystershell and give them some poultry spice as a pick-me-up.
It's getting towards moulting time when they tend to stop laying anyway, if she's starting to look a bit scruffy and losing feathers it might be this.
Are all the others laying happily in the coop ? Check for red mite if they're all starting to lay outside (although they'd be reluctant to go in at night too if this was the case and it wouldn't affect the shells)
HTH
Karen