A number of things can be causing this issue. Let's eliminate these one-by one:
1. The most common issue is that the health of your torrent is poor. To check this, try downloading a torrent from the RSS feed. Find these by tapping on the menu icon (hamburger icon) in left corner of the header then tapping on ‘Discover’. Download one the torrents listed there. The key difference between these torrents and the torrents you’ll find in the wild are the number of seeds (these are usually heavily seeded), therefore they are being shared a lot. If the torrent will download quickly, then the app is working fine, but you need to look for torrents with more seeds.
2. Your app is in ‘wifi-only mode’ and you are not on a wifi network. To check this, tap on the menu icon (hamburger icon) in left corner of the header, then tap on ‘Settings’. Turn off the switch for ‘Use Wi-Fi networks only’.
3. Your network connection is poor. Check your wifi speed with the free Speed Test app from Google Play.