Chunk-Bang? That indicates to me that the delay is not between ignition of the powder in the pan and ignition of the main charge (i.e. Whoosh-Bang), but between the c0ck fall and pan ignition. If that's the case you're not getting a good shower of sparks hitting the pan powder and a different flint will likely do the job.

The other possibility is the geometry of the lock is such that the sparks are not being well directed into the pan. But I would be surprised if that's the case with a Pedersoli lock. Can you post a picture of the lock at half c0ck, with a flint in place and the pan closed? Also, you can operate the lock in a dark room (give your eyes time to adjust to the darkness) with no powder in the pan to get an idea of how much spark you're getting and where they seem to be hitting.