Hammer spring rides on top of the trigger pin, and one leg of the hammer spring rides in the annular groove of the trigger pin to retain it.

If the spring is installed backwards on the hammer, of the spring has been tweaked (read bent to lighten up the trigger, but done so down away from the coils, then the spring leg may be being lifted out of the annular groove when the trigger is pulled back.  Also, some spring legs are a bit too thick, and they will not ride in the trigger pin annular groove correctly.  If this is the case, then either the bottom edge of the spring can be filed (semi- knife pointed, or the annular grove can be widened (drill spun and a file) to correct the problem.

But to not get a head of ourselves, lets just start by confirming that the spring is installed correctly on the hammer, and the legs are resting on top of the trigger pin.

From the tacked post at the start page of the trouble shooing forum, "Trigger/hammer pins walking out" (read tacked by myself to solve 99% of the problems, and save someone from having to ask this question again).