Eggs with thick shells

[Porterfive]

Hi,
I have a mini dome incubator, yes I know it's cheap, if you'll pardon the pun lol, but thought I'd give it a try for our first time hatching eggs.
Has anyone had any experieince with this incubator? It seems to be doing well so far, just a little trouble keeping the temp steady, especially at this time of year in Mississippi.
My main question is that I have been candeling the eggs and a few of the eggs, bantam duck wings, the light will not even shine through these shells. Does that mean the shells are just thick? If they are that thick will the chick have trouble with it when it comes time to hatch, if they ever do of course?
Some of our other eggs, you can throw them on the floor and they will not break, we give them to our dogs ocassionally, you have to stand on them to get them to pop. Is that normal for shells to be that hard? Wonder if there is something missing in their diets?
Thank you in advance for any help.
Jenny.

 

[McGoo]

I have eggs that are dark in color and candling is going to be very difficult if not impossible. I don't know if the thickness is going to be a problem. Maybe someone else does.

Good luck with the hatch.