Is this chick much smaller than the others? Is this something that came on suddenly and this chick has been totally normal to this point? What have you been feeding the chicks besides chick crumbles? Anything? If this chick is much smaller, it is likely failure-to-thrive. It needs a quick shot of vitamins, preferably Poultry Nutri-drench. There is also a good possibility the chick has eaten some pine shavings and has got an impacted crop. Feel just to the right side of its chest and see if you feel a hard lump. If you do, you need to get a teaspoon of coconut oil into it asap. Chill it until it's hard, and break off bits and place them on the tongue one at a time until you get the full teaspoon into the chick. Then massage the hard lump very gently until you feel it break up. Then make sure the chick drinks plenty of water.