I have 10 hens. Don't get 10 eggs per day very often but they are usually laid in the same nest even though there are 5 nesting spots. Chicken preferences! I have a few broken/ cracked/holes in 'em eggs every week. At first thought they weren't getting enough calcium as I Occasionally get eggs that obviously have thin shells (and these get broken more often than the regular strength shells). But they have oyster shell free choice and they eat their shells, So I don't think that's the problem. So I think it's just the Chicken thing of them wanting to lay in the same nest. At least in my case.
The funny thing is they did used to all use the same nesting box - we had one raised up and they all wanted to lay in there - to the point where there would be two in there and one trying to shove her way in. I don't remember having a single broken egg. Once I moved that nesting box down (I would've put the others up high but there was only one platform) they've all spread out evenly, but now I'm getting broken eggs.
Now that I'm typing that out, it does seem likely something is getting them - maybe a pest that can't climb up high? Rats definitely can but perhaps something else? I have also seen the roos going in the nesting boxes, but they did that when it was raised too, so I don't think it's them.
Also confirmed that it's not a larger pest animal coming in - they've been locked up the past few weeks because of a dog coming around, and I just had one break the other day.
I'll probably start collecting their eggs soon because I don't think my last hen is going to go broody, so it'll become a non issue, but I will put some rat-sack out just in case. (safe from the chickens of course)
Thanks for all the help

I'll get a trail cam before my next set of chicks, and I'll also seperate out the hens into smaller groups, so hopefully they won't overcrowd their nests as much. I've never had any success rat-proofing...anything, really, so if it is them I'll probably just cut my losses. Hopefully the ratsack will at least control them.