If either the bolt arm or the trigger/bolt spring is not broken as you stated the only other possibility is that the trigger/bolt spring is bent or shortened.
Here's the proper operation: in the fired position the bolt arm rests against the side of the hammer with the round notch in the bolt arm around the edge of the cam. As the hammer is drawn back the cam pushes up on the bolt arm, causing the bolt to pivot on the bolt screw and the bolt end to retract from the cylinder, dropping below the frame. By the time the hammer reaches full cock the bolt arm slips off the beveled edge of the cam, and the trigger/bolt spring causes the bolt to move upward through the frame and into the cylinder notch for the next chamber.
It sounds like the trigger/bolt spring is broken or bent. The long and short arms of the trigger/bolt spring should be flat and parallel, except near the ends. The short (trigger) arm will be bent downward to engage the notch on the trigger face below the sear. The long (bolt) arm should also be bent nearing the end but much more 'gently'. This longer arm rests on the two bolt arm and exerts force to make the bolt rotate about the bolt screw and push the bolt up through the hole in the frame. You should be able to see this in action with the trigger guard/grip frame removed. The longer arm might be too short, bent or just too weak from heavy usage.
Also, Pohill's suggestion that the bolt screw could be too tight is another possibility. The threads on that screw are actually just under the head. There is no bushing, so the bolt simply rides around the screw itself.
If loosening that screw (just a very little bit) is not successful, I'd replace the trigger/bolt spring and see if that doesn't correct the problem.