I have had a few frozen embryo transfers that have failed. what causes failed implantation?

Second opinion: Look for second opinion. You need to know what is your embryo stage and quality prior to freezing and what is embryo quality after thawing. Are they using vitrification to freezing your embryo? What protocol do they use for your fet? That all make difference!

Uterus and embryos: Failed implantation is common. An aneuploid embryo, with abnormal chromosomes is likely the cause. A receptive uterus requires normal structure of the uterine lining and surrounding uterus. Hormone preparation and timing must be correct. The transfer procedure itself is sometimes problematic. Cycle monitoring, medications, and ultrasound are essential. Check with an expert in rei.