We had a bug a while back where some of the music made the app freeze while rendering. Go to your music lists in the app. Scroll to the bottom. Click “download more music”. Do the same for sound effects. This will update the faulty music files.
It can take a long time before the render starts, the app starts with rendering the music and the voices so if you have a long video with a lot of music it can take a while.
The possible length of your plot varies depending on the device you are using. This is related to the size of the generated video file in combination with the amount of memory on your device. If you want to tell an epic story, try making into shorter episodes instead.
A good tip is to restart the app before you render, especially if you have worked on your plot for a long while. Restarting makes sure the app has enough memory to render.