If the droppings are white and liquid, and the chick appears to be in pain and cries out when it poops, that's most likely Bacillary White Diarrhea. It's infectious, so can infect all your chicks.
If it is the case, it can help to give a bit of honey mixed with slippery elm bark powder (which is a natural antibiotic, and a mucilage which coats the bowels and kills whatever is causing infection, while healing the digestive tract too).
You'd need to give it several times a day. We just mixed it up on a tablespoon with a bit of water, and caught the chick and got it to sip from the spoon several times, but how you get the chick to ingest it is open to various methods obviously. You could use a needle-less syringe and squirt a bit down their mouth, but it's easier to get it into their lungs than if you're dealing with an adult chook. You could also add it to their food. I would not substitute water for it though because it absorbs a lot of water and can be dehydrating for some animals, so give water free choice too rather than remove water for a day just to get it to take medicated water.
We also used Lukas' pawpaw ointment on the vent, which in our single case had become red and raw. (Only one of ours got infected and survived with this treatment and recovered rapidly).