Why does an embryo not implant after transfer?

Great question: In most cases we do not know. Some reasons: 1) poor quality, old, or abnormal embryo, some estimate only 1 embryo in 4 will be successful, 2) inappropriately developed endometrium, which can happen if too much/too little Progesterone before transfer, 3) bad placement or rough/difficult transfer. Many things we still do not know!