It sounds as if the current safety is no longer making good contact with the sear (pivoting it firmly into the hammer's full cock notch). It could be that the safety or sear are worn, it could be that the sear pin or the hole for it in the sear or the frame are worn. If the the sear and sear pin (and the holes) are okay and the sear is pivoting without excessive "wobble" or slop, then the problem might be able to be fixed by peening the stud on the thumb safety which engages the sear so as to displace some additional metal toward the sear so as to tighten up this fit.
P.S. - If you find yourself in need of a replacement safety, I have a mid-1970's blued Belgian one in the parts box you can have for $20 shipped (stud, pin, and spring installed)