When I am polishing with a powder the marble gets a dull, wavy appearance and sometimes looks rough and dimpled.  What's going on?

You are describing what is known as " orange peel".  Orange peel is the result of using too much acid on the marble.  In other words, most marble polishing powders contain acid and you are either using too much powder or working the powder too long.  To remove the orange peel you will need to rehone the marble.  When you start polishing apply a small amount and work it for several minutes, making sure to check it often by pulling the slurry back with a window squeegee.  If a shine is not being produced, fight the urge to add more powder.  First work the powder a little longer before adding more powder.  This should prevent orange peel.  If you should happen to orange peel the marble you will need to re-hone the marble to remove the dimpling.  Orange peel cannot be removed with more powder.  You will find that it is very easy to orange peel certain types of marble.  Negro marquina, crema marfil and other soft marbles are easily orange peeled.