teresa
I don't know anything about IVF but as far as I know, you shouldn't have any thick mucus in your uterus. The thick mucus that can make it hard to get pregnant is the cervical mucus and it is produce in the cervix and then exits from there. It doesn't go "up" into the uterus. My guess is that your problems getting pregnant probably aren't directly related to your CF since the embryos aren't taking.
I would look into alternative or complimentary therapies to help you on your next round of embryo transfers. Like acupuncture, hypnosis, massage, herbs, etc.
Sorry that is the only advice I have.