There may be something to the hereditary hypothesis, my sister had a retained placenta too. It may also be more common with having girls. Be warned if you do want to take angelica (also known as Dong Quai) that there may be problems with breastfeeding.
Just to let you know, 17 days overdue, with three attempted inductions and lots of oxytocin, I had a healthy baby boy with no retained placenta this time. I can't tell you exactly what it was that i did differently, but:
1) I used baby oil instead of cocoa butter (no stretch marks this time either!)
2) I took Pregnacare's own omega 3 supplements instead of Mumomega, and
3) When they told me to push out the placenta, I pushed like hell!!
All the best with your deliveries,
Kate