I am not saying that you are tied to LTP but the forum is. Polymers are what is in the grout a bit like plastic particles what happens is that your tiler has washed down but the grout has grabbed into the pours of the
tile then the polymers have risen to the top creating a skin over the grout so putting diluted acid just wouldn't work as it cannot penetrate the polymers this is where you need a liquid which I will name if someone jumps in and gives me permission this needs to be worked in and given some time to allow to work pref with a soft brush so it can get in the pours the clean off and put diluted acid on it with a brush work in give it time then rinse off and your problem will be solved. you may miss bits if so just repeat on area.
You need to understand your problem before you can solve it.