How many have you had? Just a couple other considerations...
If the lambs are not properly aligned so that the feet enter the birth canal and put pressure on the cervix, such as in the case of a breech birth, the cervix can fail to dilate. Normally the reflexes of a live lamb will cause it to align properly. However, if the lamb is dead, it will often not line up properly and the ewe will fail to dilate fully. In the case of ringwomb, it can sometimes be difficult to tell if the lamb was dead before labor began, which cause the ringwomb, or if the ringwomb occured first, prolonging delivery leading to a dead lamb. I personally have had the first scenario occur but not the second. If there was any red or browning discharge before or during labor, or if the lambs were not full term, I would suspect some kind of infectious abortion agent causing your problems. If the lambs were full term and healthy, then I would look to nutrition.