I have a forum set up in group mode "visible groups". I am the administrator of the site and made some posts in this forum. The course has only one group and this forum is set up such that it is only visible to the members of this group. For most threads in this forum the name of this group shows up in the Group column, but not for all. For some of my own threads the field in the group column is empty. This course was recently moved to a new moodle server. Since then, the threads with empty group column are not visible to the students of above-mentioned group anymore.
Question 1: Why are these threads not visible to the students of this group?
Question 2: What can I do in order to make these threads visible to the students of this group? I was hoping to find a possibility to change the group of a thread, but I cannot find it.
Remark: When I first created this course, the above-mentioned group did not exist. Maybe I created the threads with empty group column before this group was created and maybe this is the reason why the group column is empty for them. But I am not sure whether this is the real reason for their group column being empty.