I make a sitemap every month, following the instructions here:

https://help.forumotion.com/frequently-asked-questions-f5/text-sitemap-t9480.htm

I notice that nearly all listed pages (URLs) in my sitemap are over a month old, with hardly any from the last month of postings. Has anyone else noticed this? Can it be corrected?

Here is the generator site:

http://www.xml-sitemaps.com/

Try a better sitemap generator that collects every link from your forum.

Or, you can manually create a sitemap by using 30 minutes of your time.

( depending on how large your forum is )

I suggest only add the links with useful information ( that's about your forum ) in your sitemap. Good luck,