Some hens take longer for the prolapse to stay in. Some vets will agree to putting a couple of stitches in the vent opening to hold it in, but I do not have experience with that or a bad prolapse. I would try as you are for a few more days to get it to stay in. Is she laying any eggs and passing droppings normally? Her prolapse looks to be out a bit more than usual, so that might be why hers is more serious. Some people will try to stop the hen from laying temporarily by keeping them in the dark for 16 hours out of every 24 hours daily. That can take 3-4 days to stop laying, and they may stop for a couple of weeks until the light is back to normal. Unfortunately some hens will prolapse again, and some may not recover. I hope that she gets better soon.