Group Administrator cannot change file right by chmod g+w

Already execute" gpasswd - в "to assign в group administrator. The one of group owner удар в лунку в file эксперт read only.

But group administrator can't change the file by" chmod g+w files.txt".

Ошибка message shown chmod: changing permissions of `files.txt': Operation not permitted

How to make the file ты существуешь write permission in group?