lost another chicken

 if anyone can help i am in desperate need of advice
i had four lovely chickens from my friend as her and her husband have been unwell and want to travel.
our favourite died about three weeks ago and we were really upset, then this weekend we lost another.
they start of with runny poo(white?) then just stop!
they are always cleaned out, have fresh water and food everyday and always have grit. i give them greens and when i am in the garden they come out with me!
i just have no idea what i am doing wrong.
i have asked a few people who i know and that have chickens and they have just said it is one of those things
please can someone tell me if there is anything i can do, i dont want to loose the last two and would like to get some more but just not sure what to do for the best

thanks

they must be about two/three years not sure 
my friend had them from young

they have layers mash in the morning and corn at teatime. they always have a bowl of grit and fresh water.
the cats and dog are feed indoors and only eat dried food!
i always have a cabbge hanging for them to peck at so just not sure what is the problem. dont now when they wre last wormed could this be the problem!

it might be worth getting the vet to do a postmortem - if it's something infectious the others may come down with it.

also, you say they get cleaned - i would strip the entire house and run down and wash it thoroughly with jeyes. and change the soil and any bedding.
as well as cleaning their feeder and drinker.