Relays don’t propagate posts to other relays. Nostr’s design is that relays receive and send information to the users connected to that relay. However there are some relays that have been designed to blast notes out to multiple other relays, and some people have also created aggregators that gather info from multiple relays. But automatic propagation of data from relay to relay is not part of the network protocol. 
Discussion
same question when I first used nostr🤓
Initially I thought that’s how it should work, similar to how bitcoin nodes propagate blocks to other nodes. But with Nostr there is no need for every relay to have a copy of every single event on Nostr all the time, and doing so would make it harder to run a relay.
Thank you! This helps a lot