Flooring Tile - Help! Unevenly Wiped Down Fresh Grout - Any Suggestions?

Hi Everyone! We just put new grout on our newly tiled shower wall. We were working on such a large area that by the time we got started wiping all of the excess grout down, it had hardened to the point where we had to scrub extra hard. This left uneven spots all over our wall – including some spots that go below the tops of the tiles. The grout is still fresh (though dry now) and we haven’t touched it at all in 24 hours. The question is, can we just mix some more grout and lay it down directly on top of yesterday’s coat to thicken it up? Since we wiped everything down smoothly, will we have to roughen it up at all first (if so how?)? If you don’t recommend laying another coat down on top of the original, will the grout be ok if it’s only a little bit under the top of the tile? Thanks for your replies!

Do it right away and it should work for you, don't wait.:)

Thanks for your reply. I think we've waited too long at this point unfortunately, so we're going to just redo the areas we think are too thin. Good advice for our next tiling project though!