So far, but not so Good.

So my husband and I have been trying to conceive for 3 months now. I'm not on birth control and we are not using any other method of birth control. Yet I'm STILL not pregnant. My first child I had when I was 17. And it was the first time with my first boyfriend that I got pregnant. So I'm wondering how in the heck I got pregnant then by having sex ONE time yet now I have sex every day multiple times and nothing. Please help.