Coat the vent area with vegetable oil or vaseline to help soften it. Everything needs to be very gently done back there, the tissue is very fragile. Refrigerate or freeze some coconut oil until its hard, break it into tiny pieces and feed them to the chick, try to get about a teaspoon in it, just put the pieces into the beak it should swallow them. That will help with any constipation. Check your brooder temp with a thermometer. Too cold or too hot can be causes of chicks getting pasty butt. The first week it should be 95 degrees under the warmest part of the lamp (assuming you are using a heat lamp), and reduce that by 5 degrees per week. And make sure they have plenty of room to get in and out of that warm spot, so they can warm and cool as needed.