goldhound56,
The failure of the hammer to be held in the cocked position when firing single action could be caused by the sear being stuck, lazy or worn.
With the trigger forward, depress the sear (located at the rear of the trigger guard) with your finger and release. The sear should snap back under spring pressure. If it does not snap back, it may be caused by a damaged or missing spring or is dirty enough to hamper proper function.
If it returns when depressed, it is a good indication of wear on the sear or hammer. There is another reason for a nonfunctioning sear and is a little too involved to explain here.
Good luck,
Sprint