Why do sectional issues start to divide the United States between 1815 and 1829?
Sectional issues started to divide the United States in the time frame that you mention because the North and South had different economic systems with different needs.
The different needs can be seen in the Nullification Crisis that came at the end of the period you mention. This came about because Northern interests wanted tariffs while Southern economic interests wanted free trade. The different economic systems caused them to have different interests and the different interests drove the sections apart.
This can also be seen in the conflict that led to the Missouri Compromise. Here, the fact that the two sides had different interests led to conflict over whether the land of the Louisiana Purchase would be free or slave.
These conflicts started to divide the United States on sectional lines in this time frame.