Jumping in late and without reading full thread, but wanted to share my experience.
I developed a short LP (6-9 days) after miscarrying my first baby. My GP did believe me when I said I had confirmed it with ovulation sticks and temping, and ordered a couple of day 21 progesterones. The first was barely high enough and my period came very soon after, and the second was too low. On the basis of this, the fact I spotted several days each cycle and miscarrying again, my GP referred me to a specialist.
Unfortunately, the specialist did not believe in low progesterone and said you either ovulated and could get pregnant or you didn't - there was no 'weak ovulation'. I saw limited improvement with herbs, acupuncture, maca and pretty brutal diet changes (no sugar, caffeine, cow's milk). Finally, I thought sod it and ordered natural progesterone cream (Emerita, from Amazon) and started using it 3 days after ovulation at the maximum dose. I got pregnant that cycle and now have a ten month-old on my lap!
The cream is expensive (especially if you use it the whole pregnancy, like me) and neither my doctor nor complementary therapists recommended it, so I was self-medicating. If ever tried for another, I would try and get progesterone from my doctor again but if he said no, I'd do the same again in a heartbeat!
No idea why I had the short LP. A scan did show borderline PCOS, but I did ovulate each month.