How to deal with not getting pregnant again I'm 21 whats the problem?

I am a 21 year old mom of one. I want to have at least one or two more kids ,but I can't seem to get pregnant I have tired everything, keeping up with bbt, tracking my cycles everything .I have even tried de-stressing I cant get pregnant .It has been a year and a half of trying. What could be the problem? The doctor says he sees no reason I can't get pregnant again Any advice would be so great? Thanks Jen