It sounds like you aren't getting the sheet hot enough. When you
overheat PnP, it will get gooey and your image will smear. If you
don't occasionally see any signs of this, chances are your iron
isn't providing enough heat.
You might try preheating the metal in a relatively cool (225F) oven
before dropping the PnP sheet down, then rubbing a bit with a wooden
or plastic burnishing tool to get it to stick. But if you're using a
large sheet of brass in a relatively heavy gauge (18+), you'll have
difficulty getting the whole thing hot enough to transfer. Perhaps
you can cut it down?
It's also possible the metal isn't clean. Try an abrasive cleaner
for the first cleaning. Scrub, rinse well, and rub the metal hard
with a paper towel. If there is still tarnish coming off, it's not
clean enough. Pickling sometimes helps.
Finally, the laser printer may the problem. Some toners simply don't
work for various reasons. If this is the case, run off the image at
a copy shop.