egg shells too hard/thick

[Xashi]

I'm not entirely sure, this is only my second batch of duck eggs. But the shells seem way harder/thicker than normal and my ducks seem to be having a tuff time hatching.

What causes this? And how do I fix it for next time?

 

[Xashi]

As a side note we have one breach baby, that needed an assist. Seems to be doing fine now that he can breath!