The most important issue left unresolved was there was no definition of who a citizen of the United States was. Well until the 14 amendment was ratified in 1868 stating that any person born or naturalized in the United States was a citizen.
slavery
At the Constitutional Convention, the most significant disagreement dealt with the issue of representation in the legislature. The convention was held in 1787.
Represntation
Constitutional convention deadlocked over the issue of representation. The delegates did not seem to agree on the selection of representatives to the Congress.
The fundamental issue that was raised at the constitutional convention was slave trade and human rights abuses. Those were the issues that were strife then.
The Fourteenth Amendment establish equal rights and protection to all United States citizens. It covered citizenship, protection, legality, and equality that hadn't been an issue at the Constitutional Convention.
The Constitutional Convention
The taxes
control of commerce
The Constitutional Convention dealt with slavery issue in a conclusive manner. The addressed the rights of the slaves and their right to own property among other contentious issues.
representation of the states in congress
How the president should be elected.