Are you using ACV in your chicken's water? If so, that could be your culprit. ACV has been used for some time by those who wish more female offspring...worked on my hatches, worked on my meat/4-H show rabbits and seems to work in the professional horse racing community...they've been using this technique for years also. I've even read about a sheep farmer that has an 85% return on ewe lambs since starting to use ACV in the water.
thats interesting to know
i just got 5 litres and got a load of breading pairs
may try it an see what happens