Low progesterone at 8 weeks

Kind of discouraging with my progesterone. I’ve been getting blood drawn since 5 weeks pregnant due to low progesterone at first is was 12 at 5 weeks, then 8 at 6 weeks, then 6 at 7 weeks. According to my lmp I am 9 weeks but had an ultrasound yesterday and I measured 8 weeks. Last week when I thought I was 8 week my progesterone went up to 10 so I thought we were on the up side. Well I got blood drawn yesterday at what I thought was 9 week but turned out to be 8 weeks got results today and now im at a 6 again. They put me on 200 mg orally once daily when they first seen it was low and I’ve been taking it once a day and with results today they said start taking twice daily. I just don’t understand why they won’t give me anything stronger. Baby was healthy heartbeat of 137