When you get to the end of a module, it's important that you close the window that you are completing the module in and giving it a few seconds to update. This is the same whether you are completing on a computer, tablet or mobile phone. When you are at the end of a module there will be a message to close the window so you will know when to do this.
If you press the home button or the back button your module won't save and you will need to do it again.