Tumbled marble and grout problem

Hello,
I installed black tumbled marble in my kitchen about a year ago.  The grout
is a dark gray.  The floor is now very dull now and the grout is especially
bad in certain areas - white/chalky.  I'm wondering what I can use to help
revitalize both?  **The grout is a particular problem.  If it were all
uniformly dark gray the floor would probably look much better overall.

A little background.  An independent "professional" installed the tile.
Afterward, he put down an enhancing sealer (can't remember the brand, but it
was sold by Floor and Décor and used on the display tile that I chose) to
give the stone a deep rich look.  But -- he didn't apply it correctly.  He
put it down thick and left it to dry.  I'm not a tile installer so I never
questioned this- I thought he knew what he was doing. Well, it was like
someone melted a candle on the floor - awful!  Sticky, waxy - just a mess.
Turns out that the excess was supposed to be wiped up!  Of course, he
wouldn't return my calls to come back and fix the problem.   The company
that sold the enhancer told me the only thing to do was strip the floor. I
did this as best I could with a brand called TileLab.  Afterward, I applied
the enhancing sealer again following the directions.  Floor looked okay, but
never as deep and rich as the display.  Anyway, the grout started looking
spotty 6 months later.  Not everywhere -- just randomly around the floor.  I
have tried cleaning it, putting down more sealer, etc.  Nothing seems to
work.  What causes this chalky look?  Is it related to being stripped at
all?  I was assured this wouldn't hurt the tile.  Would love some advice on
the grout and on ways to pep up the marble.  Thank you. -Teresa