trying to become pregnant

Oh why this is happening an nothin does happen..... ok I'm 36 going on 37yrs old I have 3kids 17yrs old daughter, An two boys 6, an a 3yrs old son. An I'm trying to have at least one more. But it been a year ago  I had a mistcarrage on first time an it's been a year I been keeping track of all different apps from my phone of my menstral period an ovulating an do show of system of implantation I been doing a home pregnancy test an all show - negative 😞 I also see my doctor an did pregnancy test an blood work an everything is ok I even my doctor told me just keep taken prenatal vitamins an just keep trying to have sex often which I have no problem is that I just don't get it. have this ever happened this to u????