I hope you find that it has come out this morning.
I'm always surprised that vets do not manually remove the placenta after a delivery like that! Here, the vets that I use, do. I had a dystocia here a couple of weeks ago. After removing the foal, my vet removed the placenta. My neighbor had a mare with a dystocia night before last. We took her to the vet hospital. Again, after delivering the foal, the vet there removed the placenta.
There are so many things that can go wrong with leaving a placenta in a mare that I like to see those come out no more than 3 hours after foaling. As per my vets instructions, I will apply gentle traction to the placenta twisting it counter clockwise until it is removed. I always have my vet flush and infuse a mare anytime the placenta has been retained more than three hours.
Good luck with your mare!