It could be many many things.... maturity of the egg released, whether or not there even was an egg released,the quality of the lining for the fertilised egg to implant in, the acidity and general quality of the cervical mucous at the time of intercourse, the quality of the sperm, whether or not there are enough progressive sperm with no defects, whether or not the sperm can reach the egg (think tubal blockages or endo) etc etc etc. It is a wonder anyone falls pregnant at all!

Most doctors will tell you there is only a 22% chance of falling pregnant each month. TTC takes time. Sit back and enjoy ( or lie back, lol... )

By the way, I am very sorry to hear of your miscarriage . Its a tough thing to go through. My thoughts are with you and your angel bub.