A very quick and dirty analysis of the page code shows there are two instances on this page of references to
http://www.photobox.co.uk .... they are insecure links (http not https) so the whole page gets the label "not secure"
The index page has no such references, as it has no images ....... so remains as secure
If you wish to check other pages ..the procedure is and I'm using the "new" Edge:
right click a blank area of the page - choose "inspect" ( or hit ctrl+shift+I)
across the top screen that appears on the right, choose "Console" ..
You should now get a list of all the issues the software detects with the site code .... it will highlight security issues, often as a code 17 .. but sometimes with just an exclamation mark in a yellow triangle.... the narrative will, usually, read something along the lines of, but can differ:
Mixed Content: The page at 'https://www.omegaowners.com/forum/.............' was loaded over HTTPS, but requested an insecure image 'http://www.............................'. This content should also be served over HTTPS.HTH a little