A 32-year-old woman from Mumbai attends the fertility clinic with a 2-year history of secondary infertility. Her menstrual cycles are regular at 28 days. Basal body temperature (BBT) charting over 3 months shows a biphasic pattern with a sustained 0.5°C rise beginning on day 14, confirming ovulation. However, a postcoital test (PCT) performed 6 hours after intercourse on day 12 of the cycle reveals motile sperm in the cervical mucus. Serum estradiol on day 12 is 180 pg/mL (normal preovulatory: 200–400 pg/mL). What is the most likely physiological explanation for her infertility despite confirmed ovulation and sperm presence in cervical mucus?