Davy: If the frizzen is made out of a oil hardening steel like the Siler, L&R, Davis and Pedersoli (based on Coyote Joe's comment), it is hardened clear thru.
If it once threw good sparks and it stops, I would look at the flint, or the possibility that some oil in on the frizzen face or the edge of the flint.
In other words, I wouldn't worry about Case hardening it.
If the frizzen was made out of a high carbon steel like 1095 it is also hardened clear thru and recarburizing with something like Kasenit will not help it.
If the frizzen is from one of the inexpensive locks which was made from a low carbon steel then re-carburizing it will improve its sparking.
Before considering recarburizing a frizzen, I would use a file to see if it was soft. If the file can't cut it, it should work with the right flint setup.