jdb puzzle

I need to step through some complicated code in the javax.swing and
javax.swing.tree packages using a debugger, but I have not been able
to find a debuggable version of the jdk-1.3.1 distribution.
Therefore, in a desperate move, I made a new rt.jar file including the
classes of interest recompiled with the -g flag. (I included print
statements in the sources prior to recompilation, to serve as
indicators that these were the class files that were actually being
loaded by the debugger.) I started the application under the jdb
debugger and set breakpoints in the recompiled jdk code, right at the
newly added print statements, but jdb ignored these breakpoints. It
did not complain or announce that it was stepping over these
breakpoints; it just silently stepped right over them. (The printing
by the inserted statements happened as expected.)
The question is why does jdb behave in this way?
Thanks,
KJ