and since your an experienced flintlock shooter, your sure the flint is well secured in the jaws and the right distance from the frizzen. Also sometimes it take a few strikes in a new flint to "set the flint" to where it chips off in certain places and then begins to spark nice and long.

Another thing you can try is flint the flint over. Some flints like bevel side down instead of up. Also make sure all the oil is off that new frizzen. Take some alcohol and really scrub it and the flint down.