There were 55 delegates representing twelve states present at the Constitutional Convention, but only 39 of them actually signed the U.S. Constitution. There were three different groups of people who perhaps ought to have signed it, but didn’t. There were those Founding Fathers who did not attend the convention at all, those who went but left early, and those who disagreed so whole-heartedly with the proposed Constitution that they refused to sign it.

Three men attended the Constitutional Convention, but in the end did not sign the Constitution. Those men were Edmund J. Randolph (VA) George Mason (VA) and Elbridge Gerry (MA).
Randolph, who was the governor of Virginia at the time, introduced the Virginia Plan along with the other six delegates from that state. The Virginia Plan was chiefly responsible for expanding the purpose of the Convention from simply amending the Articles of Confederation to reconsidering the entire federal government. It proposed, among other things, the three-branch federal government we still have today consisting of the legislative, executive and judicial branches. Randolph declined to sign the finished Constitution because he felt that it had deviated too much from the Virginia Plan.
George Mason, also from Virginia, kept notes of the Convention debates. He decided not to sign the Constitution because he came to believe that a central government would not represent the states and the people well, and that it would ultimately result in the kinds of government that other countries had, the kinds our Founding Fathers were trying to prevent - monarchies, aristocracies, tyrannies.
Elbridge Gerry, a businessman from Massachusetts and one of the signers of the Declaration of Independence, refused to sign the Constitution for similar reasons to those of George Mason. He too believed that the rights of the states and the people were not adequately protected under it.
Two of the biggest surprises in American history are the two founding fathers who did not even attend the Constitutional Convention. Thomas Jefferson was representing his country as the United States Minister to France at the time, and John Adams was in Great Britain serving as America’s first ambassador there. Despite their absence from the Convention, both men were able to contribute their ideas and support for the development of the new federal government, Jefferson through letters and Adams through his work on the Massachusetts state constitution and the work he published while in London, A Defence of the Constitutions of Government of the United States.
There were thirteen delegates to the Constitutional Convention of 1787 who simply left Philadelphia for various reasons and they never returned before the document was completed. Those delegates were:
Of all the delegates who were not at the convention or who left early, there were only three who refused to sign - Edmund J. Randolph (VA) George Mason (VA) and Elbridge Gerry (MA).