How old are your chickens?
When is the last time you wormed your chickens?
Are they laying eggs?
Molting?
Are they eating well?
Since you have more than one, with the same symptom of diarrhea, you should have a fecal float test done. You need to rule out internal parasites. Worms or coccidia can cause diarrhea. Any vet can do this for you. Or, if they send out the specimens to a private lab, it's just a matter of including your sample. There are also state labs and colleges that perform this test, at a nominal charge.
Other things that can cause diarrhea in chickens, are, eating watermelon, too many raisins, molasses, milk, intestinal infection from eating dead animals, crop problems, reproductive illnesses, just to name a few.
Diarrhea in itself can cause other health problems for the chicken. If the feathers cake up with feces, Vent Gleet or Fly Strike can happen. So, keep the vent area clean. A simple flushing using and irrigation bottle, with soapy water, will remove the poop.
If you don't have a vet to help you, you'll have to be a medical detective.
If you can upload some photos of their poop, that would be helpful. The members here at BYC, will be better informed to help you.