Are the messages disappearing quickly (within minutes or hours) or after a longer interval such as days or weeks? Many mobile devices only show recent messages (such as those received in the past few days or month).The time interval for display of messages and discarding old trash or spam messages can usually be changed by the user in a setup menu for that app on the device. For example, for the default email client on the Galaxy S III this is set up as follows:
Period to sync Email: Choose a period of time to maintain synchronization between your phone and email account.
http://eguides.sprint.com/support/eg..._settings.html
Bill