I can't get pregnant!

Hello me and my husband have been trying to have a baby for four years now. He has a son and I have kids to. But we can seem to get pregnant we have tried tracking my period, ttc kit, stop drinking period, jus bout everything besides in vitro. And I did some research and found an article tht said if we both have kids thn there's probably not a problem. I don't know what to do does anyone have any ideas or suggestions?? I could really use some help.