Downloaded movies

Hello,downloaded movies that I have purchased from Apple won’t stay downloaded on my 128gb ipad.They keep unloading with out any action from myself.Downloaded music is ok.Films on the Amazon and google apps are fine, It seems Apple have a problem with this it’s their device their movies that wont work for off line.My ipad is backed up to the latest iso 13.6 update.Has I have just read a previous person with this same issue back in 2014 was advised to get the latest update to remedy the problem.I have had this same problem with this and other iPads and in the past just give up.Its poor for Apple to have this problem.