Why is my newly done grouting rock hard in some places and crumbly in others?

[Shitemum]

I went over the builder's grouting months after they originally did it as there were holes in it.
Some of the re-done bits are fine and others crumble when I run my nail over them. Why? (I did them all with the same lot of mixture).

[flightattendant19]

Was it mixed properly? Sounds like there was some moisture lacking in places maybe.

[jeminthecity]

Grouting on floor tiles?

Did it dry out too quickly/over hot pipes?

[bodiddly]

It does sound as if it wasn't mixed thoroughly!

[Shitemum]

Thanks, yes maybe I didn't mix it very well was a bit hasty and there were little lumps......

[scrapes it out and starts again...}