What gives?!

After three years and three failed IUIs we started IVF. 7eggs retrieved, 4 fertilized, 3 day 5 blastocysts and finally one perfect embryo. Implantation went great and 9 days later, a positive blood test! HCG was at 200. Then, went today for a follow up and HCG has dropped to 10. They are ordering a repeat test and I’ll go back Saturday for another blood draw. So incredibly frustrated and disappointed. Can anyone share if they had a similar story and if they were provided any reasons this occurs or ways to be more successful?