By any chance, did you get the eggs from someone here on BYC? If so, could you PM the user name to me? I hatched out three sebbies I got on here too. And while I don't see any signs of mycoplasma in my flock (YET) and they are almost 8 weeks old, I am seeing a lot of other issues in my flock, predator attacks aside.\
Although I wouldn't put fertility into the ring of issues so easily. Yeah it may be related, but EVERYONE seems to be having fertility issues this year, after the bad winter, and drenching spring. I figured between reading everyone's issues with hatching this year, losing my own chickens to predators (which aren't normally this bad), losing LOTS of chicks to mysterious death (I have five at the poultry lab now, just waiting on results), and the other issues here and there, that this entire year is just a wash out.
Skeeter's death was my last straw, where I decided to give up on pretty much everything until next spring.