help! Issue with 3 week old chick pooping

My chicks are 3 weeks today (WOO) and I have been trying to introduce grit into their diet. They go out daily for about half an hour and rummage in the grass. One of my chicks, a rhode island red has been having trouble pooping... she will ruffle up her feather, make a quick repetitive chirp and strains. the movement is much larger than usual and will be kind of stuck coming out of her vent until she manages to get it all out. I have incorporated apple cider vinegar, but I feel like this is happening because they are rummaging outside and are not being given grit. while not trying to poop, she is happy, hanging out with her fellow chickens and acting normally. This also doesnt occur every time, just every now and then, but when it is a regular poop, she still makes a quick chirp like when she is straining. That being said, I integrated some into their food as well as giving it in a small container on its own. the chicks have shown ZERO interest in it. eating around the grit and just knocking over the other container. I need some advice in dealing with both issues! please help!