Detaching Placentas

We are having a rough calving year this year.  Today we pulled our 3rd dead calf.  All 3 of these calves should not have died, and we don't know what the problem could be.  In all three instances we monitored the cow closely, all three were slow to progress, but we intervened within 2-3 hours of the water bag showing.  One of the cows was a first calf heifer, the other two were young cows that had calved before.  All 3 calves were presented correctly, and the mothers were not in distress.  We pulled these calves because we wanted to avoid calf problems because of prolonged labor, not because we were particularly worried that the calf was in trouble.  With all three calves the placenta delivered with them.  There is no necroses in the placentas, no abnormal bleeding, nothing,  just calves that have been dead long enough to not respond to resuscitation.  Two of the calves were normal sized, about 75 pounds.  One was very large.  We are wondering if we are missing something in the diet that is causing this? We only calve about 25 cows, so we don’t think it’s a fluke.
Any thoughts are appreciated