I used a bull on heifers one year and he was great used him again on the same heifers as 1st calvers and the next group of heifers and he gave me problems. The 1st calvers had larger calves the second time around, fortunately they calved without assistance but the heifers had all kinds of trouble, particularly the slightly smaller individuals.
Looking back the biggest difference i could see, we had a better season the second time around meaning bigger calves.They weren't a great deal heavier but they seemed to be a little more open in the shoulders, which brings me to my next point.
A lot of people with more experience than I are telling me that calf shape is as important or more than birthweight i.e shoulders and hips etc. So i think the calves being a little larger making their shoulders a little bigger made them difficult to pass.