Difficulties TTC #2

I am having issues with low progesterone on day 21 cycle even after taking Leztrole? Feeling so down when I got the results .. 1st cycle was 19nmol/l, 2nd was 15.4nmol/l and 3rd was 10.1nmol/l . Why is it going lower and lower :( I conceived my #1 on the 1st cycle 2yrs ago.. now it seems like it's so so difficult..