I have had the same issue trying to make some bunny fart round and not using enough powder. I think the problem I had is with the large case volume of the 38. the small volume of powder either moved forward or spread out along the bottom of the horizontal case and did not reliably ignite causing a noticable funny ignition sound. When I lift the barrel between rounds the powder reliably ignite with a nice little report.
I am looking for a less dense powder with more volume per weight to fill the case more. going to try trailboss I think when I can find some.