Pending means the sub hasn’t gone through to the home instance of the community. If you’re the first subscriber, this means the there will be no inbound federation bringing the content from that community to your instance.
If someone else on your instance has already successfully subbed, the federating is already occurring, and your instance will be receiving the activity as it comes in.
Your instance will then show it to you, both in your subscriptions and in general, even though the sub is pending.
If your sub stays pending, you may have to unsub and resub to get it to work. If no-one else on your instance has subbed either, then the activity will continue to not show up for as long as it is pending.