M.G.
Hi A.-- It's been a while since I have cleaned grout and I know it can get pretty nasty.
It sounds like it's spotty and that could mean mold spots.
I would try X-14 extra strength if possible. It's strong so turn on the fan and maybe wear a mask. Let it sit a while then go back and scrub. You'll be surprised at how very little scrubbing you'll have to do.
This is the product I used long ago from Home Depot. (For my kitchen) Unfortunately, it has been so long I don't remember the exact name. However, I do remember finding it in aisle where they sell tile, grout and tiling tools etc. It was in a yellow/gold mustardy color bottle and it was specifically for cleaning grout.
Also, my new favorite product is EASY OFF BAM - It's the best for clearing up hard water spots on glass or anythihg for that matter
Hope this helps!
Elaine