So sorry for delayed response. I am no expert and so wanted to dig up some information that I personally found particularly informative to quote to you. Here it is:
Progesterone levels after ovulation (midluteal, middle of the second half of the cycle) in a nonpregnant patient are generally at least 8-10 ng/ml (25.44 - 31.8 nmol/L). Much lower levels usually mean you did not ovulate. If you are pregnant, progesterone levels are usually at least 10-12 (31.8 - 38.16 nmol/L) ng/ml to have a better chance of a good pregnancy outcome and most doctors like to see progesterone levels around 16-18 ng/ml (50.88 - 57.24 nmol/L) or more though there are many successful pregnancies with lower levels.
Of course, now you are probably thinking the worst as the above info ways prog should be at least 25.44 and yours was 20.5. But a progesterone test is usually done mid-luteal phase. If the cycle in which you got your blood tested was 34 days, and you ovulate 14 days before AF, then your progesterone would peak on day 27. So when you got your levels tested at day 22, the progesterone probably hadn't yet reached it's peak.
Actually, longer cycles are associated with increased fertility, and our cycles shorten as we head towards menopause. Mine can be as short as 24 days, so from an objective viewpoint, your cycle seems very healthy and fertile - I am jealous! I have had 5 failed IVF's and am sure that my light short cycle is making things difficult (although our primary diagnosis is that my husbands sperm is 100% abnromal).
Of course, all this is no reassurance when it doesn't answer why you have not fallen pregnant. I take it that you are just at the beginning of the process of getting tested. When do you see your specialist to get some answers?