Try checking the permissions for the group to see if the update permission is allowed on the field.
To do this:
1) Go to Tools - Customize - Groups and Permissions
2) Highlight a group in the list that is having troubles and click the Defects tab at the top right
3) Expand the Defect tree item, then expand the Update tree item.
4) Look for the fields in the list and see if they are checked. If they are not, check them (you likely will not need the ''By Owner Only'' checkbox.
5) Once checked save changed and have the users log back in to check if the fields are editable now.