lemmy.ml is constantly overloaded. They are asking that people sign up on other instances. If you want to see content from lemmy.ml, just subscribe using your lemmy.world account. However note that even the federation of content from that server is spotty, as the server is regularly getting hammered.

The lemmy.ml instance is run by the developers of Lemmy, who are currently heads down laser-focused on the upcoming 0.18 release which makes a number of large changes to improve performance for all instances.