RSS Feed not updating

first, make sure your feed validates: https://validator.w3.org/feed/

Yup it validates. I mean it works in readers and stuff for whats there. But its showing very old posts and only 6.

Have you tried the following troubleshooting steps:

-deactivating ALL plugins (yes, all) temporarily to see if this resolves the problem. If this works, re-activate them individually (one-by-one) to find the problematic plugin(s). If you can’t get into your admin dashboard, try deactivating via FTP or SFTP or whatever file management application your host provides. If applicible, also remember to deactivate any plugins in the mu-plugins folder. The easiest way is to rename that folder to mu-plugins-old.

– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems (because the hooks remain unless plugins completely removed or some plugins stick around in cached files. So by renaming the folder, you break them and force them inactive).

– To rule out any theme-specific issue, try switching to the unedited default WP Twenty Twelve theme for a moment using the WP dashboard. If you don’t have access to your admin area, you can switch to the default theme by renaming your current theme’s folder in wp-content/themes and adding “-old” to the end of the folder name using via FTP or SFTP or whatever file management application your host provides.

I had tried the plugins stuff, but not the theme. I swapped the theme and the RSS feed changed, but still showing older posts.

I swapped the theme and the RSS feed changed, but still showing older posts.

Try clearing your browser cache before viewing

To clarify, it was a different set of older posts. They were even older. Would using Feedburner help at all?