Femara/Letrozole

Madison • 27 & Married. TTC#1 with Endo and PCOS
Anyone have experience with this medication working one cycle but not the next? Last cycle was my first time using it and on days 12-14 I had terrible pelvic pain and positive ovulation test on day 13. By day 15 my pain was gone. Progesterone level on day 21 confirmed I had ovulated. 
Fast forward to this cycle- I'm on day 14 with very minimal pelvic pain and negative ovulation tests. Can't help but worry it won't make me ovulate this month. Has anyone experienced this?