So sorry for your loss. I was given misoprostol for a missed miscarriage as well. It seemed to "work" but 3 weeks later I was still spotting. An ultrasound revealed that I had retained "products of conception" and ended up having a d&c. I wish I had the d&c in the first place but that's a personal decision. Good Luck.
Sometimes, it just doesn't. Especially if this is not your first child, the uterus likes to hold on a bit more. I hope the final dose does the trick.