Is the loose stool only occasional, and is it still excreting solid poop? If so, that's normal. Chickens have two 'types' of stool, since they can't exactly urinate. If that's not the case, then I'm unsure.
Welcome to BYC. Most chickens can have a "cecal" poop about every 9 times they go, and those are runny. However, with young chicks, be on the lookout for symptoms of coccidiosis, which is very common in chicks under the age of 5 months. This can cause runny poops, sometimes bloody or full of mucus, and become very sleepy and lethargic. Here is some info about it: http://www.the-chicken-chick.com/2012/12/coccidiosis-what-backyard-chicken.html