Dirty Grout which can no longer be cleaned - we had this problem and we used
Grout Shield | Grout Color & Sealer | Grout Restoration System. At the time we used this product there was no one in the area doing this work so my otherhalf did everything and it was a massive job, but this many years later I can tell you the floor and grout still looks like new and all I do is wash it. The grout has been scrubbed clean and has never changed. Incredible product for those that have old floors that need help.
If anyone is interested I will be happy to give them more details if they pm me.