I don't know what else you may have already tried, so I'll just throw out some tips and you catch the ones you like:
* verify that you do not have .nomedia files in the directories where your files are. These may be hard to see. With ES File Explorer, for example, you have to select an option to "show hidden files" to see files starting with a period.
* Go to Settings-->Storage-->Unmount SD card.... then use the same to then mount the SD card again. This causes the system to detect the card again as if you had just inserted it. See if this triggers the media app(s) to scan your SD card again as if it was just inserted.
* if you have not powered down the tablet completely by using a 2-3 second hold of the power button to pop up the power down window, do that. The power-up system reboot will cause all services to restarted, and storage volumes remounted, which may cause devices plugged into the unit to be scanned.
* factory data reset. It's really not that bad, and is a very useful feature. I do one periodically to clean up the system, restore filesystem perfomance to new (like defragmenting), etc.