Hcg slow rise after threatened miscarriage?

So I got my bfp 7/18. I started bleeding 7/19. Went to the ER 7/21 where I was diagnosed with a threatened miscarriage. My hcg then was 62. I followed up with my ob/gyn 7/26 where my Hcg was 38. They told me it appears I am miscarrying and to come back in a week for another lab draw. I went yesterday 8/1 and my Hcg rose to 45. Anyone have any idea what could be going on?