I just subscribed to a new community but I am only seeing one post. Is there a setting I need to change to see older posts on the newly subscribed community?
You need to specify how you are browsing the community because different UIs/apps have their own ways of displaying content. Are you on desktop or mobile? If desktop, what front-end are you using? If mobile, what app are you using? Check the post sort settings to make sure you're not filtering out a specific time period. It's also possible that there really is only one post in the community.
If you're the first person from your instance to subscribe to that community, you'll only see new posts. If there is a particular post or comment you want to reply to, though, you can grab the URL and put it into your instance's search box to sort of force it to federate across.