Trying to get pregnant

Both me and my boyfriend are 26 we've been trying to have a baby for almost three years and haven't even had a scare. I do have a 3 year old with a previous ex boyfriend in the past. I'm just wondering for me to be so young why is it so hard for us to have a baby. I figured it can't be that I can't have kids..I already gave birth to a baby boy who is now 3..anyone have any advice?? Or is or was experiencing something like this in there relationship.