I find Lemmy isn't super at pushing newer content at the top, and you have to just mix between the different sorts. The content is definitely there. The alternative way to mitigate this is as follows:

• In my settings I have "show read posts" unticked. This ensues posts I interact with (e.g click on, vote on) with disappear from my feed afterwards (upon refresh/reload, of course). I don't think it clears posts that have appeared in your feed but you haven't interacted with. This is extremely effective in keeping your feed fresh.
• When browsing on my phone (Boost For Lemmy), I make frequent use of its "hide all read" button to quickly gut anything on my feed that I've seen before (no interaction needed).
• You can, of course, just manually hide posts as you go but uh, the above methods are better at automating that process for you.

I used to get really annoyed with stale feeds but upon doing the above it's pretty much always fresh content.

That said, it won't hurt to subscribe to more communities!