Why can I not get pregnant after 8 months of trying? Me and my partner are healthy I know when I am ovulating so we always have sex around that time.

Stress. Stress can be a major factor when couples are unable to conceive when they have been otherwise able to do so. The stress hormones affect all the other hormones of the body and can lower levels of fertility. Just relax and let nature do her work.
Many reasons. They are many different possible reasons. Some of them include- possibly there is something wrong with your partner's sperm, he can have no sxs and have abnormal sperm. Could be something wrong with your tubes- could be blocked. Your hormones may not be right and many other reasons. You should go for an infertility evauation and he needs one sperm analysis.
This can be normal. It is estimated that successful conception of pregnancy is around 20-25% each time a couple tries per month. However, a couple is not defined as infertile until unsuccessfully trying for at least 12 months or more consecutively. Should that become the case one should see their doctor.
See an expert. There are many reasons, structural and hormonally both in you and your partner! See a reproductive endocrinologist if you are not pregnant in the next 4 months.