Welcome to the forum, Teresa.
We've got some contradictory advice going on here and should back up a bit.
We must know how wide the grout lines are in each of the areas and if the grout is unsanded (smooth texture) or if the grout is sanded (rougher, sand-paper texture). Unsanded grout is meant for grout lines up to and including 1/8". Sanded grout is meant for grout lines from 1/8" up to 1/2" (depends on particular grout brand and I've not heard or used your brand before). If unsanded
grout was used in gaps larger than 1/8", that's a problem.
Measure your gaps and report back. Realize that polished marble tiles have a small bevel on them. Examine some of your scrap polished marble tiles to see what I mean. Ignore the bevel when measuring so that you get the true distance from the edge of one tile to the next.
Rich is right about using a dental tool on unsanded grout. If you use the kind of metal dental pick used by dentists to clean teeth, it's pointy enough to scratch away unsanded grout without much problem. But scratching off grout with your fingernail is another story.
I see three possible things going wrong to create soft grout:
1) The first is what Paul said...the grout was mixed so soupy that it weakened the grout,
2) The grout is defective (possible, but not likely), or
3) Due to hot temperatures, direct sunlight, or a fan blowing in the room, the grout way too fast and made for weak grout. (Cement needs time to dry or you will effectively "kill" the grout's microscopic structure.)
Now, before we go too far, I'll tell you about an obscure problem/fix that we've experienced for soft grout. This isn't a cure-all and may or may not apply to you. Some folks have had success firming up their soft grout by getting the walls/grout wet, allowing to dry, then repeating this sequence 4, 5, or 6 times. This isn't some crazy scheme, but rather a suggestion from SOME of the grout manufacturers. This has been somewhat successful by some folks. But before you jump in with both feet, YOU NEED TO CALL YOUR MANUFACTURER AT THIS POINT. Find out what they say.
And realize that you have to have at least a one year warranty by either the general contractor or subcontractor (whoever you've wrote the check out to for the work). Leaving the customer with soft grout (if that's what you indeed have) isn't acceptable and would fall under an issue that needs to be addressed by the responsible party. If there indeed is a problem, it falls on their shoulders to correct the situation. This shouldn't cost you a penny to resolve this. Is there any problem contacting them?
Oh, and finally:
Because you mentioned sealing the grout within the context of this problem, I'll offer up some bit of fact that is rather widely misunderstood about sealers. Sealers are only meant to buy you time if the grout is stained with something. It's meant to help you clean off a stain that would otherwise penetrate too deep that would prevent you from cleaning it off. Sealer isn't a waterproofing or thought of to aide in keeping water away.