Several times it happened that answers to posts were not shown. When I looked into it the problem was an access level I did not use in the whole forum. That means that either the users changed the access level (what I do not believe, because it happened several times) or it was changed by chance.
To prevent that users mix up the access levels in their answers I suggest that the superusers can deny usergroups by configuration to change the access level of their answers on their own. On default the access level of an answer to a topic should be that of the post/topic.
This suggestion corresponds with the following problems and feature suggestions which all intend to improve the rights management/access levels: