I'm licensed in California although this is a summary of the support laws in your state which I understand to be Maryland. http://www.ncsl.org/issues-research/human-services/termination-of-child-support-age-of-majority.aspx
It sounds like you've waited too long. Child support past the age of majority is usually voluntary and can be done with the written consent of the other parent codified in a court order. I would suggest that you speak with a family law lawyer licensed in your state as to how to proceed.
Under current Maryland Law, child support continues until: age 18, graduation from high school but no longer than age 19.
Once she turns 18 and has graduated from college the other parent does not have a legal responsibility to pay support anymore.
Maryland law only provides for child support until the age of 18, graduation from high school, however, not past the age of 19. Therefore, since your daughter graduated from high school, her father can not be ordered to pay child support. College is not considered a secondary school.