Trying to conceive

I’m 18 & my boyfriend is 19 we have been ttc since we first got together which has been a few years and I still haven’t gotten pregnant we have sex literally all the time and nothing’s changed we have even used preseed while ovulating and nothing works. I don’t know what’s going on it doesn’t seem to be to normal. My brother had sex with with his girlfriend once and got her pregnant but nothing is working for me. Any advice ?