Ovulate 1st month on 50mg of clomid but not the 2nd

Kenya
I have pcos and my doc put me on 500mg metformin and 50mg clomid. I ovulated normal on cd 15 the 1st month. I'm on cd 19 my second month of 50mg clomid. I'm not even close to ovulating. All my opk test was completely negative. Has anyone had this problem?