I got my answer from the History Channel
Although 55 delegates participated in the Constitutional Convention, there are only 39 signatures on the Constitution. Fourteen men, having already left Philadelphia, were not present for the signing, and only Delaware delegate John Dickinson had a proxy sign for him. Three delegates—Elbridge Gerry of Massachusetts and Edmund Randolph and George Mason of Virginia—were dissatisfied with the final document and refused to ink their signatures.