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 using this site: https://checker.openwebtorrent.com/. The key difference between a torrents with poor health and a good torrent is the number of seeds (these are usually heavily seeded), therefore they are being shared a lot.  

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’. Uncheck the box for Use Wi-Fi networks only’.

3. Your network connection is poor. Check your wifi speed with a free Speed Test app from Google Play.