hard time conceiving

Hi I'm christie I'm a mother of 2 kids. With my 12 year old she wasn't planned. But with our 3 year old we tried one time and I got pregnant. We are trying again.  We tried last month (i think it was a time that I was not ovulating) and then again this month of the best day for me to get pregnant and I took a test it's negative.   I've been having the cramps tho and I thought for sure I was.  I'm not sure what I'm doing wrong.  I thought for sure I would get pregnant right away just like I did with my 3 year old.  Any advise/tips would be greatly appreciated!  Thanks