TTC i need help..

Okay so I've been TTC for the past couple of months and I have no idea what I'm doing wrong.. My period is usually always on the 20th or 21st. This past period I had was may 27th which I was really weirded out because I always have regular periods but clearly it came.. Now the question I have is, am I supposed to be having sex before my fertile window starts or during or both. Im gonna buy a ovulation stick this time just to try and do it right right because I'm beginning to get depressed about this. Everyone else is getting pregnant I feel and me and my husband have been trying with no luck. And I know we can have children cause we do have a almost two year old.. Help a girl out pleaseeeee