What can cause grout to crack?
You made the mixture too thin and runny. Make up a new lot and keep it thick and almost like a thin dough. Then work the mixture into the cracks with a plastic scrapper and try to finish with the grout at almost the level of the tiles. Also you should have laid the tiles onto a 6mm plywood base if you have floor boards in the bathroom. Before doing all this I suggest you scrape out some of the bad grouting. I have been a tiler for 20 years.
