A common reason for this issue is that the user has logged into Mahara through Moodle, which will automatically set up an account. Then they upload information into the account.
Then the user accesses Mahara through another method (not through the same Moodle account). This will create a new Mahara account, and the user may think they have lost their data.
Resolution: Ensure user logs into Mahara EXACTLY the same way they did when they uploaded the data.