One of our chooks has stopped laying completely

We have 5 chooks. They are all about 15 months old. Recently one of them slowed down egg production and has now stopped altogether. Sometimes she sits on the nest as though to lay an egg, but no egg appears she used to be the naughty, noisy adventurous leader of the pack, but now shes hanging out on her own alot and stays quite close to our back door often even when the others are out scratching. Also seems quiet and off her food She's not moulting, not broody and I don't think she's egg bound - everything looks normal there and I can't feel anything bulgy.

I rang the vet - but they couldn't offer much advice. Its $50 for a consultation. Can anyone offer any words of advice? Our chooks are our beloved pets who we love dearly. I've got 3 kids who are starting to worry....

Thanks, Suzanne