I've been pregnant four times all with my being on top. I don't think it's the position.
If he is getting his ejaculate into your vagina, position is not a problem. As you have been trying now for a year and a half, I would recommend seeing an infertility doctor. There are a series of standard tests that you as a couple should consider at this point. These include a sperm count, an X-ray of the uterus and fallopian tubes called a hysterosalpingogram (HSG) to see if the tubes are open, and some hormone blood tests. About 1/3 of the time, these tests discover a problem with the man, 1/3 of the time they discover a problem with the woman, 1/3 of the time the tests all come back normal. There are treatments for all three scenarios. After a year and a half of trying, you are completely justified in seeking out a physician to start an evaluation for you.