It's easy for an old guy like me to miss details but nowhere in these two threads do I see any reference to exactly what you are grouting.
Nor do I see what type of grout; sanded, unsanded, tint added, etc.
Your symptoms are classic old/exposed package of grout or incorrect mixing ratios, water/air temperature and water impurities.
In other words, not reading and understanding the directions, implicitly.
You keep mentioning something I didn't know still existed; tile store.
I recommend the Big Box stores as people here are aware.