Thanks for the replies. I did put in fresh wonderboard behind, so the tiles should have a firm footing. From the replies I think my error may have been in the application or the drying. I used a pre-mixed grout, but I was using a damp sponge to try to clean up after myself as I was applying it, so I'm thinking maybe that had the effect of diluting or thinning the grout? Also, would a space heater to hold temperature constant during drying and a regular light misting during the drying process help?