Available Offline problems...

I have only recently started using Memrise and am having trouble using the ‘available offline’ setting. First of all, when I download a course to my Iphone the download stops and will only resume if I turn off the screen and reenter the app. The bigger issue I’m having is that when the course finally finishes downloading, it appears to erase the local course data and start downloading everything all over again. The only way I can keep the course data on my phone is by disabling my wifi connection.

Does anyone else have this problem?