New knoppix 5.0.1 installed over old RedHat, and most stuff works, but
the LaserJet 4L printer doesn't. The recommended driver (HP LaserJet
4L Foomatic/ljet4) is installed, the Print Manager window says it's working,
and I printed a test page. It's queued, but nothing happens. The printer
is (still plugged in, and when I press and hold its button, it prints a
test page that looks fine, so it has paper and is ready to print. But the
print job's status is "Processing..." and stays there. "lpq" says the job
is active, and stays in that state.

So what clues am I missing? Is there a diagnostic tool that will tell me
what's going wrong?

The CUPS stuff seems to work. It even found a network printer on a
nearby Mac, and is able to print to it successfully. But that only works
when the Mac is awake and running; I'd like the knoppix desktop box
to use its own attached printer.