Can't get pregnant

I'm 24 and I already have three boys that are 4, 3, and 18 months. This is the biggest I've ever been. I'm 5'7 and I weigh 245. I have a pretty regural period. With my first three kids I got pregnant right away. I took ovulation tests and I was pregnant the same month. This time it's just not happening. My weight is the only thing that's different, but everything I've read says if your ovulating regularly your weight shouldn't be an issue.