HELP! Sidebar disappears on pages with text

I’m currently creating a template for a sub-site based on the template of my main website. When i add a page/post the page messes up and the sidebar seems to be the problem.

For example: https://pulsepulse.co.uk/festivals/?page_id=1 or https://pulsepulse.co.uk/festivals/ are fine but https://pulsepulse.co.uk/festivals/?page_id=4 is not.

I’ve checked almost everything, tried removing the sidebar etc – no luck.

Help?
Thanks!