If the compromise was to ban slavery north of the southern border of MO, why then were MO, KY, VA, MD, & DE all slave states?
It pertained to the western territories, rather than states that were already in existence.
The Missouri Compromise forbade slavery north of 36'30" N in any new states formed from what was the former Louisiana Territory, not in existing states. The chance of any ordinance banning slavery in states where it already existed was less than zero.
SOURCE: http://memory.loc.gov/cgi-bin/ampage?collId=llac&fileName=036/llac036.db&recNum=155