debugger won't engage

[Mikey]

I have a script for a particular object (on mouseMove) with a debug
checkpoint set in it at the top of the handler.  However, the debugger
never kicks in (but the script runs).  I have tried clearing and
setting it, but it never kicks in.  I have set a checkpoint in another
script, and the debugger always kicks in.

What should I be looking for when trying to determine why the debugger
won't kick at the top of one handler but will for another?  Again, the
script executes, but the debugger never appears.

-- 
On the first day, God created the heavens and the Earth
On the second day, God created the oceans.
On the third day, God put the animals on hold for a few hours,
   and did a little diving.
And God said, "This is good."