Can you post a pic and what sample ? The sample on the sample board from the maker of the grout or what? I have always had good luck with using the sample board so please explain.

if you haven't already added a grout sealant, just spread more grout over top and continue the process as you would.

" just spread more grout over top and continue the process as you would. "

That does not work. You cannot just add grout over the top of grout, a layer that thin would be guaranteed to crack and fall out.