It's most likely that your sear is disengaging, but that the hammer lobe has galled to the mainspring and won't slide as it should. It was just a poor design: might have worked better if the hammer was carbon steel instead of 420 stainless. Could be made to work better if you stoned the contacting areas of the hammer and mainspring to a smooth polish and then had the hammer hard chromed or Metalifed, or some similar kind of plating. Then, polish the hammer lobe again after plating; but don't polish through the plating! You likely won't find replacement springs except from a dead Pocket 9 somewhere.
Good luck, ...Deeton