[Ponyman]

Hi Mike ~

There is no spring to "tension" the thumb safety lever - resistance is provided by a small "bump" on the back (or inner) surface of the lever, which rides on the surface behind the lever and falls into the detents at the extremes of the lever travel. Cycling the safety lever thus results in an arc being scratched on the surface. It sounds like the "bump" on your lever was either worn down from excessive cycling or perhaps removed by a previous owner. Another possibility is that the lever is bent outward just a bit.