Thomas Jefferson was not one of the founding fathers. During the convention when the constitution was signed, he was still in Paris working as the French minister, which is political position. John Adams was still in Europe serving as the minister to Great Britain.
Many other people did not sign. Samuel Adams, John Hancock (he did sign the declaration of independance) and Patrick Henry turned down an invitation to attend because they thought it was going to result in another monarchy, just like the one they were running away from in England.
Only 39 out of the 55 people attending signed it.