Hi, sorry to hear about your problem. You mention they are on sand which like you said could be the problem. Because they are on chick crumbs they really don't need much grit at all as chick crumbs are very easily digested. My first chicks never got grit until they joined my big hens and started eating grains (which is where they really need the grit). The only thing I can suggest is getting some olive oil into them to help unblock it. Also put some oil around the vent even inside if you think you can do it without damaging the chick. I used a cotton bud once to try and push a chicks prolaspe back in, the cotton bud fits in the vent and doesn't damage it. So you could try soaking the cotton bud in oil and gently putting inside the vent, but not too far just the very tip, only if you think you can it is a very delicate procedure. My chicks prolaspe never got better but it's still healthy and running around now 9 weeks old. I know it's used for chickens which are egg bound. I would remove the grit too. As long as they are on chick crumbs and greens they won't need it. If your worried you can give a little once a wek or so as it is stored in the stomach for some time. Other than that I don't know what to suggest. I'm afraid if the oil doesn't work and they are in a lot of pain you may have to make a tough decision. Sorry I know it's not nice but I hate to think one of my chooks is suffering. I really wish you the best of luck and hope they get better. Hope someone else may have some ideas.