I agree that you need to switch to a good grower/starter or layer feed depending on the ages of your chickens. Follow the directions on the bag as far as when to switch over to what feed - it varies slightly from brand to brand.
On the other hand, there is a wide range of 'normal' for chicken poop. Sometimes it's firm, othertimes more runny, sometimes brown, sometimes lighter, sometimes really really smelly, sometimes not so bad.
An occasional runny poop I wouldn't be concerned about at all. It might be some plant they free ranged on that is making the bowels a bit looser.
What you NEVER want to see is blood in the poop - that would indicate a potential infection or parasite, OR completely watery poop that persists.