some users cannot comment on a post while some can

Within the timeline for any group, it seems some users cannot comment on a post by certain people, but others can comment on that same post. That person who is restricted in some places is able to comment on other posts within the same timeline. We would like it so that if anyone can comment on a post, everyone is able to comment.

I assume this restriction is based upon the usergroups of some users versus those of others. I don't know where the settings are that blocks some users from commenting in certain scenarios. Please tell me where I can change the settings to allow everyone to comment on any post.