Some posts and comments doesn't sync to other instances.
I noticed that some of my posts and comments in the community I created (!skypictures@lemmy.ml) doesn't seem to sync to other instances.
For example, if you go to the community through lemmy.ml and lemmy.world you won't find the welcome post through lemmy.world. But the post I put just after that one do appear. The same issue also seem to occur with the sidebar.
I have seen similar issues with lemmy.ml communities from another instance. I believe the cause is server overload at lemmy.ml. Looking at the Lemmy source, if another instance fails to receive a post, it will be retried in 6 seconds, then 1 hour, and finally, 2.5 days. With the great Reddit migration, expect delays in federation until things settle down.