Lost NDS games?

Hi all,
I am new to the forums, so apologies if this topic is posted elsewhere...I have downloaded most NDS games for my son and successfully put them onto his TT, however a couple of times the games have just gone missing between checking on the computer that they are actually on the TT and turning on the DS to play them.
I have checked back on the computer again to find that they aren't on the TT anymore...
How does this happen, and what can I do to fix the problem?
Any assistance would be greatly appreciated :)
Thanks,
Mel

Well, maybe the NDS file is hidden or maybe your memory card is corrupted. I would suggest making a backup copy of what ever things is inside your memory card and re-format it ;D

Thank you, in the process of doing that now :)