Are these young hens? What breed? Was there anything wrong with the egg itself? And what and how are you feeding them?
Fragile eggshells can be harder for a hen to pass than good, hard ones. If that's the case, you might only have to supplement calcium and vitamin D.
Overeating can lead to obesity, which makes eggs harder to pass. Overeating can also give the hen a lot of nutrients she doesn't need. Giving those nutrients to the egg would make it more likely to ensure a healthy chick, so her system might just do that, without any consideration of how hard it might be to pass that egg. In this case, limiting feed might be the solution.
Young hens can just have problems as their bodies adjust, and some of the more productive cross-bred brown egg-layers (Read: Red Sexlinks) can jump into laying a little earlier than their bodies are prepared for. I try to avoid those birds.
And a common statement (can't find any scientific studies to back it up, but it makes a great deal of sense) is that a light schedule that isn't constant can mess with their systems too. So if you're providing supplemental lighting for egg production during winter, try and make sure it's as regular as possible.