We seem to be having trouble with a handful of the larger lemmy instances, as well as some misc other instances even though these instances aren’t dead-dead: https://fedia.io/federation (at bottom of page - “Dead Instances”).
I thought maybe it would’ve been a problem with a specific lemmy version, but there are several versions in the list and also some still-living Mastodon instances that are running reasonably up-to-date versions.
I see lemmy.ml is on that list. Checking on a fairly active community there (c/linux) it looks like it got posts from yesterday but none of the 10 most recent ones from the past 22 hours are visible on fedia.io.