Hi Boo's Mom,
I'm in same situation as you only have been doing 200mg Prometrium 2X day for 6 unsucessful cycles. DD BFs 2-4 times per day. I am leaning toward Clomid at this point. Some REs believe that Clomid is the only really effective way to address LPD because Clomid does it at the level of the Corpus Luteum where progesterone originally comes from. Plus there is added benefit of increased egg quality. We are afraid of twins but probably going to try it. For what it's worth. I've been on B6 (B comlex) for years.
Some have tried Vitex, but I had LPD even while on Vitex. But I will say it had no adverse affect on my mild supply.