Why haven't I got pregnant

So I have 3 kids with a previous man. My husband has 3 with a previous lady. We've been married a year and a half, decided to TTC. Been trying 6 months now. Why is this taking forever for us? Took a month for each of my kids, a year for his first and a month for the 2 after. Any ideas? Conciveing tips?

Adding In: The kids are his lol. I'm 21 and hes 24