I was perscribed Clomid as a part of a regiment my Dr put me on. Per my Dr he found that I wasnt ovulating. I had been diagnosed with PCOS years ago and was always told by my previous Dr that my irregular and lack of periods was due to PCOS yet after changing my Dr and a few test in May he found that my Progesterone levels were low. So this 1st cycle started with Progesterone med and then the Clomid.
Let me guess - your doctor is an OBGYN?
You didn't answer the rest of my questions - what other testing have you had and are you being monitored? And again, that CD21 progesterone level means nothing if it wasn't done 7DPO. The only way to know you're not ovulating is by charting your BBT or having ultrasounds. If you weren't doing those, it is possible the bloodwork was just done on the wrong day.
ETA: I just saw your next message. Has your husband/partner had a semen analysis? Have you had CD3 bloodwork or an HSG?