A 28-year-old woman from Delhi presents to the gynaecology clinic with a complaint of irregular menstrual cycles and difficulty conceiving for the past 8 months. She reports that her cycles vary between 24 and 40 days, with unpredictable timing. On examination, she appears healthy with normal BMI. Pelvic ultrasound shows a normal uterus and ovaries without cysts. Serum progesterone measured on day 21 of her cycle is 2 ng/mL (normal luteal phase: >5 ng/mL). FSH and LH levels are within normal range. What is the most likely physiological abnormality underlying her presentation?