Hello! Make sure that you 100% load lessons while you are online, and they will later work properly in offline mode too, both audio and text. All the lessons which are completely loaded in online mode are available in offline mode too. Make sure not to clear app’s cache in the meantime.
Offline mode is definitely working on Android. Its most likely that you are still connected to a wifi network that is very weak or you can’t have access to or maybe when you disconnect the phone goes to mobile connection and that is also not working in certain areas with weak coverage. If there is any connection the app will try to fetch the data from the server, and in a bad connection this can take a long time, or even fail. Please make sure you are disconnected from a network to use the offline mode. One other possible culprit is that you don’t have enough remaining storage space for the app to store any offline data.