Most often that is caused by trying to display the forum in quite a narrow column space which is itself dictated by your WordPress theme. Trying one of our more ‘premium’ themes like Reboot might solve it out of the box anyway – else you will ned to make some adjustments to the column widths.

This FAQ explains this procedure:
https://simple-press.com/documentation/codex/faq/troubleshooting/why-do-columns-appear-underneath-each-other/