User changes are not applied

In my site, I have set edit options for a node "n" for a group "g" . When I visit that node "n" with another group, I cannot see edit tab and when I visit that node with group "g", I can see the edit tab. So far so good, but things get weird when I edit the "n" with a user from "g". When I finish editing and then push the submit button, web site returns to the front page but none of the changes made by the user are reflected to the "n".

By the way you might suggest giving "administer node" access control to the "g" but it won't work, because when I give "administer node" access, members of "g" will be able to edit nodes even if they do not have "edit" access in the node access module.

Do you know a solution for this issue?

Thanx in advance

Rifat Aras