The average young, fertile couple has about a 25% chance of getting pregnant each month. Things that hinder your chances include the woman being too thin or too heavy, not getting enough sleep, being stressed/sick, not having sex during ovulation (mid-cycle), and obviously, infertility. The general rule of thumb for a couple in your age range is to try for a year before seeing a fertility specialist.
I've read many of your posts and you seem to be fairly conservative in your values, so I'm curious as to why it's you and your girlfriend trying to get pregnant as opposed to you and your wife. Are you planning to get married? Not judging, just really curious about the value system you've got going on.
well, first off.you two need to be absolutely sure this is want you want and are both in a point in your lives where you are going to be financially stable!
but anyways, sex is sex. if you guys aren't using anything and its not working, both of you should go to the doctor and see if everything is working right. you could have a low sperm count or she might not be producing enough eggs. also, when you guys are done doing it, have her put her legs together and in the air for five minutes or so. this does help. if it comes down to having to use fertilization pills, be aware you could get twins or triplets or even more! it took me almost a year to get pregnant and we weren't using anything. good luck to you both!
p.s. it has nothing to do with which position you two are in either, or who is on top or bottom.