For one, the older you are, the less fertile you are. You are a lot more fertile at age 17 than age 22, but that doesn't mean that you can not get pregnant. It just means that it might not happen the first time you try. Fertility usually lasts until menopause around age 45-55.
Also look at the time of imtercourse around 2 weeks after your last menstrual period as the time you are most likely to ovulate and thus the time you have the greatest chance for getting pregnant. Again, each woman's cycle is unique, so there might be some days discrepancy one direction or the other. Secondly, you might be taking some pills or supplements that makes it harder for you to become pregnant. Are you taking any steroids or weightloss medications?
And in that line of questioning are you so thin that you don't ovulate anymore? You need at least 9% bodyfat to be able to ovulate and menstruate, and preferably you need to be above that.
Is there any irregualrity in your menses? Have the type and timing of your periods changed over the last couple of years? Have you started experiencing unuual pain with menses or with intercourse? Have you been on depo-provera shots for birthcontrol (it can take up to one year after stopping the shots before normal fertility returns.
Also, has your partner been exposed to unusual chemicals, is he wearing briefs instead of boxer shorts, and does he spend a lot of time in the hot tub (all of the above can affect male fertility.
Does either of you smoke?
All the above factors can affect pregnancy, but most likely, if you try for a couple of cycles, there is a good chance for you becoming pregnant since you were able to do so in the past.