I'm with you on TTC in my 40s and waiting for my prince before I got married. I had one kid easily at 38 and then it took 3 years before I got pg with DD#2.
Yes, your HCG level could possibly just not be high enough yet.
If you continue having problems and feel at the end of your wits with traditional medicine, I recommend acupuncture. I had 3 acupuncture treatments for infertility and then got pg last July. I continued with a few more treatments because he could help sustain the pregnancy. I went to Dr. Brad Woodle Advanced Sports & Family Chiropractic at 127th and Metcalf in OP, KS. ###-###-####
(I also went to him for chiropractic adjustments throughout my pregnancy, and at the end of it, went to stop the contractions I was having that were keeping me awake at night. I had nothing but success with every issue I asked him to address. He's very knowledgeable, but not so arrogant. I asked him to do some research on stopping the contractions and he said wanted me to come back in a couple of days, after he had the chance to do so.)