I have three kids already and my boyfriend has kids too but me and him has been having sex without protection but i haven't got pregnant yet and its been going on for 6 months now what can be wrong?

Sperm test first ...: 6 months isn't too long trying, but i'm sure if you're 24 years old with 3 kids you're used to getting pregnant very quickly. Your boyfriend may have a lower sperm count than before - start with a semen analysis for him. If it's normal you should get checked for blocked tubes with a hsg test, especially if your kids were c/sections, and then for low egg supply if you get to 9 to 12 months ttc.

Maybe bad luck: 6 month is a bit short to be worried normally, but given both your histories this is certainly a change. So it is appropriate either to wait a few month months and see what happens or to have a fertility evaluation to see if anything specific can be identified. Many women and couples have an easy time becoming pregnant at one time and have a difficult time later one - it is not uncommon.