Odd laying behaviour

Hi, I hope someone can help.

I have 4 hens, one (Bella) started laying about 2 months ago and laid most days for about 3 weeks.  Then a second hen (Millie) started laying but the Bella stopped.  Millie then laid most days for a week when she stopped and hen 3 (Flo) started.  Bella has got a bit of a cough which might explain her lack of egg production but Millie appears to be in fine health.  Is there any reason why only one would lay at a time?  Any advise will be gratefully received.

Chris