What could be wrong please? ???

I'm six days late with no pregnancy symptoms and negative test results. In September my man and I tried to have a baby and we had sex in my fertile window till ovulation. I was 7 days late and my period finally came on the 7th day. We tried again in December 2016 on my first day of fertile window and I'm now 6 days late without any pregnancy symtoms and a negative test result. I'm confused! !! What could be wrong please?????