As I mentioned in my previous email, if you this forum is set to the group mode "Separate groups", then students don't have access to posts not tied to a group they belong to.
So, although as an administrator, you can submit posts that are not tied to a group (when selecting the "All participants" element in the droplist), students in groups actually do not have access to these posts.
Maybe there should be a change in the naming of this element, as in this case it does not convey the true functionality. Maybe a developer would comment on this.
All you can do in this case is send a different post for every group, manually. There's an issue concerning this in the issue tracker: MDL-19670.