I lambed a ewe a few weeks back, Got the first lamb out with a leg back, so had to invade.
Checked her about an hour later after a trip to the vet for a couple of injections and when I got back she hadn't dropped the second one, so I lambed her again, but this time the lamb was dead.
Checked for a third lamb while I was at it and found nothing.. Two days later she was still dragging the afterbirth around and I went back in and found another dead lamb.
She is fine now even after all that and the lamb is doing fine. Her number is 13 and I'm afraid she will be going on after she rears the lamb.
The rule is check why the placenta is holding, it may be around another lamb. Don't cut it.