Why can’t I get pregnant

So with our first born it happened first try and we only tried twice that cycle. We were only 17 at the time. Now we’re 22 and trying for number 2 and it just won’t happen.. we tried 6 months so far. We’ve both been tested and are fine and I do bbt every month so I confirm ovulation. So if I’m ovulating why can I get pregnant? This is so frustrating because I was hoping to be 6 months prego right now but still trying. This sucks!!