clomid late ovulation

Hello, all! I've been using Glow for a while now but it's my first time posting. I've been TTC for almost a year - went off birth control pills in January, was diagnosed with PCOS in April and started metformin. Then added clomid in august. First and second cycle at 50mg and now third cycle at 100mg. I went in on CD 14 and follicles were only 10mm or smaller so RE started me on another 100mg right away. Went on today which is CD 24 for an ultrasound and the biggest follicle was only 15mm. RE wants me to come back two days later and give me trigger shot if follicle is mature.