How to configure a sticky site wide Confluence banner

[infrastructure]

Hi,

We have a site wide banner for our Confluence Data Centre instance as per this page https://support.atlassian.com/confluence/kb/how-to-add-a-site-wide-banner/ and configured 'At beginning of the BODY'.

I'm pretty sure this used to be sticky but we are now finding that when you scroll down a page the banner disappears.

Does anyone know how to make it sticky?

Thanks :)