Are you trying to view the files from windows or from the tablet?
We need more information, but with out it, here are some thoughts:
1. there are no files on the SD card.
2. the SD card is not formatted fat32.
3. the sd card is not being shared as a disk drive, so the files can't be seen on windows.
4. the sd card is being shared as a disk drive, so the files can't be seen on the tablet.
5. try a different SD card, if possible.
I would try turning the tablet off, waiting about 10 seconds and turning the tablet back on. then see if the problem still exists.
It seems that every tablet has a different firmware build number, so they have different bugs.
I just thought of this one. If you are using the installed file manager app, you have to select the third icon counting from the left, with the "SD" on it, to view the files on the sd cards.
Also, under "Settings->Storage" make sure media scanning is turned on for the cards.