There are at least three possibilities, all requiring the attention of your doctor:
The first possibility, if your chart does not show a clear ovulation pattern, is that you did not actually ovulate where you believe. You may have ovulated later or not at all, which would mean that you are not really "late" and that would explain why you are not getting positive results on your pregnancy test. If you consistently do not get a clear ovulation pattern on your chart, you should talk to your doctor and bring your charts along so that he or she can help you determine what might be going on with your cycle.
You might be getting false negative results on your home pregnancy tests. Ask your doctor for a blood test to determine whether or not you are pregnant.
There might be something besides pregnancy that is keeping your period away. Your doctor can help you see if this could be the case.