Hi, There could be several possibilities ranging from irregular contact period, anti-sperm antibodies, blocked tubes, irregular or failed ovulation, other hormone defects, abnormal anatomy of the uterus like bicornuate or septate uterus, luteal phase defect and general infections leading to faulty implantation or fertilization. Male factor also needs to be assessed once again. The fact that your husband has had children in the past no doubt rules against infertility, but, it should always be kept in mind that the quality of the semen cannot remain the same always. Hope this helps.