I have a 4 day old chick that is straining to poop. There isn't any dried poop or anything blocking the vent. I put petroleum jelly on everyone's rear yesterday as a few had pasty butt. This particular chick has some dried poop on the umbilical cord, but it is way below the vent. I can see him squat and strain, and his vent throbs but nothing comes out. They are on medicated chick starter, have clean fresh water with some probiotics and electrolytes in it, and have a Premier heating plate in their brooder. They also have access to chick grit. Since his vent is not obstructed, why could he have trouble pooping?