Slavery.
Step-by-step explanation:
Although the Three-Fifths Compromise encouraged the South to ratify the Constitution, it didn't solve the problem of slavery; instead, it made the problem continue to be an issue.
Both the North and the South wanted representation in the House of Representatives. They both argued how this representation should be counted though. The South wanted to be able to count their slaves as their population, meaning they would get more representation in the House of Representatives. The North did not believe that slaves should be counted towards the population, as the South saw them as property and not as people.
The Three-Fifths Compromise made it so that three-fifths of a state's slave numbers would be counted towards the population. This was good for both the North and the South.