It HAS?? But that's impossible! There's no such thing as a "frizzen" -- at least not during the late 18th century (see May 1996).
On the other hand, if your hammer is no longer giving you a spectacular, blinding shower of sparks, you definitely have a problem -- one that MAY have nothing to do with the hardness of your steel!
Unless your hammer was poorly hardened in the first place, it is highly unlikely that the temper has gone out of it. Most of our muskets simply don't see the heavy use that would bring this condition about.
On the other hand, a poorly stored musket could be the source of your woes! When I first got into the hobby, some wise individual told me to always keep my musket at half-cock because; "it's safer and better for the spring". A year or two back, I found out that this is simply not true. In fact, in his 1776 edition, Cuthbertson states:
"When the firelocks; are not in use, the cocks should always be let down, as nothing more effectually weakens the main springs, than constantly keeping them on the half-bend: (p. 92)".
Last season, my Bess began mis-firing and giving me trouble -- in fact it was hardly throwing any sparks at all. I immediately figured I needed to "re-steel" or get a new hammer. Then someone else borrowed my Bess for a demonstration and commented on how incredibly easy it was to cock. A quick comparison with the other Besses around camp proved this to indeed be the case. It appeared that my mainspring was the culprit -- it had grown weak after years of being stored on the half-bend.
Since replacing the mainspring seemed cheaper and less hassle than finding, fitting and hardening a new hammer, I decided to try it to see what happened. I put in a new mainspring last February and I am pleased to report that not only does it now take two hands to cock my piece, but it also throws sparks like nobody's business!
So, if your hammer has ceased sparking, consider putting in a new mainspring.* It only takes a small amount of filing and fitting to install one in the Italian-made musket (I haven't tried it on a Japanese model). With a minimal amount of skill a few simple tools and a little patience, chances are you will have your Bess throwing sparks like new (you'd better start shopping for period sunglasses)!
*Mainsprings for Besses, Charleville's, and many rifles can be had from Dixie Gun Works for around $25. You will also need a mainspring vise -- ask around first, someone in your unit is sure to have one.
For advise on dealing with a too stiff mainspring, see John White's article.