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).

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

Grouting on floor tiles?

Did it dry out too quickly/over hot pipes?

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

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...}