any ideas ??

so my boyfriend and i had been trying to have a baby for about 2-3 months and im confused as to how im not pregnant yet ? i was on birth control but i had stopped taking it a few months before we started trying. i smoke but i feel like that wouldnt stop me from getting pregnant all together, i mean i know it affects it but still. we were having sex all the time and he was cuming in me everytime we did. im very confused and curious , anyone know anything ?