How clme Matilda lost her powers?

No she didn't. The narrator said "Almost never" after saying that and Matilda is seen using her powers.

I think the point was that she never had to use them..."well, almost never" :)

She lost them in the book though.

It's explained that her powers came from the extra 'brainpower' she had. After Trunchbull is ousted, Matilda is moved up a few classes and has more advanced work. Because she is exerting herself and being challenged more than she was in Miss Honey's class, she loses her powers as she is using all of her brain.

She didn't lose them exactly, the narrator (Danny) said that because Matilda has a happy ending so Matilda doesn't have any reasons to why she should still use them, well unless something happens