Sounds like the single action sear is worn, its spring is broken, or the hammer notch is worn. The result is that there is too little force required to release the hammer so it falls onto the hammer lifter (transfer bar) rather than falling properly.
Many guns depend on the muscle of the trigger finger for things to work right. If the trigger pull is hard enough that the finger keeps pulling after the sear is released, all is well. But if the pull is very light, the sear and hammer lifter will interfere with the hammer and the result is what you have,