lessons keep disappear when i need them

Why the lessons I downloaded disappear when I don't have internet access.  I travel very frequent, mostly to China, spending around 14 hours on the airplane.  It is a perfect time to learn chinese lesssons.  But there is no internet connection on the plane and most of my lessons just disappear.

Is there anyway to keep it longer?  It takes a lot of time to download those lesssons and they just disappear.

As soon as I have internet, some of them come back.