I had this issue initially when my own instance, specifically with Lenny.ml.
In my case the issue was related to my subscription status. On the remote community does it show you as subscribed or subscription pending?
I showed subscription pending for a few hours, then I finally unsubscribed and subscribed again, and that time the subscription seemed to work correctly and commends started flowing to my instance.
Not quite the case.
When a user on instance B subscribes to a community on instance A, instance A begins to send in real-time the posts and comments of that community to B, which keeps a local copy of that community.
If instance B has 10 active users subscribed to that community on A, they’re all loading it from instance B. The end result is instance A only had to share each piece of content once with instance B, and instance B further shares it with the ten local subscribers, reducing the load on instance A.
The only exception is when instance B only has a single subscriber to instance A’s community, in which case replicating the entirely of the community is more work then that user just browsing it directly on instance A.
Tl;dr it’s most efficient for a large Lenny instance if most of its active users are on other instances.