Thick egg yolk

I need help. We've had a terrible time this winter with yolks getting
thick ( sometimes even
thick enough to hold and squeeze ) in our small, commercial, brown-egg
layer operation. We thought it was just the temp., as our houses are
unheated, but the problem seems to be persisting even now that the
weather has gotten warmer. This is our third year of production and we
have not had this problem before. I saw a series of postings on "Extra
thick yolks" from 2004 that seemed to be close to what we are going
through. Can anyone help?

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