I am so sorry that your U/s didn't have better news. I am new to this board and with only 3 losses under my belt can not even begin to imagine how you are feeling with 14. I don't think i would have had the strength to even try as many times as you have.
Your current story sounds a little bit like my last loss where my betas kept going up, but slower than they should after about 5 weeks. They did several u/s and at 8 weeks i was measuring 6...and they kept telling me my dates were off. At one point i said to the dr. "please explain to me how i could have a positive blood test 7 days before this baby was conceived?" that shut him up pretty quickly. sorry didn't mean to high jack your post.
Anyways, don't give up yet, there is still hope that this one was a late implanter or on his/her own schedule. You may be surprised at your next u/s. And if not, lean on us to get through it.
Good luck! SBV's headed your way.