I have just spent three of the worst days of my life sawing out the top 1/8" or so of sanded grout between the 2" tiles on my shower floor. Ugh! Replaced the grout, let it set up for two days, with intermittent sprays of water.
Now, the same thing is happening as happened originally - there are spotty patches of yellow or even orange/brown stain coming up through the grout after using the shower!
When I removed the old grout layer, all appeared to be dry, but the original installers must have put some bad stuff down under the tile! About a quarter of the tile, by the way, makes a "hollow sound" when you tap it. Sigh.
Could there somehow be the wrong tile-setting material underneath? I hope somebody can help me out - because if this keeps up there's only two options: rip out the floor, or tile over it, which I'm about ready to do.

Many thanks!