are you using a bloom formula? How much nutes are you feeding the plants? What are there current ppm levels?

If all the strains are doing the same thing then I doubt its a strain issue. I hear that sometimes different strains can turn yellowish towards the end. Sounds like not enough of the right level of nutes to me. You will need to feed them more as they begin to produce fruit. I think 6 weeks is about 2/3's through the process. Figure 9-12 weeks for a proper flower is about right. They will pack on more and more crystals in the final weeks.