Couple of options I think,
1) Where she was nesting was where all our ducks sleep at night which hasn't been a problem to her until the very last week in which our 2 Muscovy boys have suddenly become very interested in the opposite sex, the day the ducklings hatched and before I could partition her off I caught a red-faced teenager trying to have a go (clumsily at that) with his mother on the nest, this resulted in eggs being rather forcefully pushed around etc. (maybe the reason we were getting eggs rolled out the nest earlier on and we didn't notice) and I noticed one egg that died had some cracks in it that were not there when set or candled earlier. He got told off properly!!
2) While the others were hatching there often seemed to be eggs around the edge of the nest that weren't getting much warmth that we kept trying to push under her with some success but I suspect some got a bit chilled, she was fine the next day though
They all looked almost ready to pip internally to me
