A shot of Lutalytes will help her clean. Ask the vet clinic for more detailed info, but personally, I usually don't worry too much about it as long as the cow doesn't act depressed - loss of appetite, hunched up, etc. Looks like hell, but mine all eventually clean and very low precentage of open cows in the fall.
For what it's worth, since all the breds here are now fed a loose salt/mineral ration developed for our selenium deficient area, I haven't noticed many cows retaining their placentas. Sounds like you have a decent program going. Our region has undergone a drought the last two years, so feed rationing has been an ongoing challenge this winter (at one point hand pailing 1100lbs of barley a day to supplement the hay and straw).
Not to offend the Hereford folks (I've got a few of them myself in the herd, dandy cows), but one trait that I've heard that the Hereford breeders have worked hard to rectify is the tendancy of the cows to have weaker uterus' and more prolapses. From what I understand, this was related to the same gene that gives Herefords more tender meat (muscle tissue is perhaps not as 'strong' as in some other breeds). Anyway, that's all just heresay, you understand. Not saying it's the Bible.
Good luck with your cow. Let us know how things turn out for you.