Please help

My fiancée and I have been trying to conceive for about a year and a half now, still hasn't happened we have been doing it a lot lately and still nothing I'm not sure what's going on. If anyone could please give me some advice or tell me your story that would be super helpful. We do have a daughter together and she's 3 1/2 and he has a 9 year old, we have been together for 5 years now. I'm not sure what's going on but could use some advice/input.