The discovery tab is the last to load. So that could be it. Or if you are using relays that limit in the amount of subscriptions or filters, they will cut the discovery tab requests :( 

You can also try moving the app to the background and back to the front again. That forces the restart all connections with relays. If this restart fixes it, let me know. 

 how long do u recommend me leaving it running in the background the next time this happens ser? I'll try to remember it