Alright, now I feel incredibly stupid....
Finally got back from holidays and got organised enough to open the eggs that are past their due date. The eggs did have yolks!
Please don't ask me how I could not notice the yolks when candling the eggs, I have no explanation for that!
I am quite sad that 13 eggs from three different females were all infertile. I think my boys are definitely too young and the only one who is an adult is unfortunately the smallest from the males, so he probably did not get any action. I still have 5 eggs in the incubator, but this seems to be quite unlucky season.