I had sex on ovulation day with my wife 10 days after her period on her ovulation day and she is not pregnant.

What is your question?

Even healthy, fertile couples don't always conceive immediately.

Are you both sure she was ovulating? Have you been tracking her cycle and ovulation?

How long ago was this? Have you confirmed she is definitely not pregnant or is it possible that she may have just tested too soon?

It does not happen every time

My wife got her period on 11/09 and we had sex on 21/09 which was supposed to be her ovulation & fertile day. I released sperm twice but she did not fall pregnant. She has a 24 day cycle and she got her period on 04/10.

Just because she is ovulating that day it does not mean she will get pregnant that day. You should try and sleep with her a day or two before that and a after her ovulating day. Sperm stays in a woman's body up to 48hrs after you sleep with her and it can still attach to the egg