This tool is great to see when remote instances will attempt to send activity to you and how far behind you are: https://phiresky.github.io/lemmy-federation-state

There's an exponential backoff, so sometimes it can take hours before you start receiving activity again, so it's nice to know when to expect it.

I believe there is a exponential backoff mechanism in place if an instance cannot push to an other's inbox, so other instances may not have any attempt left to retry if it has been two days. In my experience un-subscribing then re-subscribing to an instance's community was enough for it to send posts after a few hours.

That is strange. Check the logs show anything out of the ordinary.