I can move all music and pix on my LG tablets internal storage to the SD card, but not any mp4 Videos! Nowhere is there the option to MOVE! These are precious recent family wedding videos I need to preserve, besides they are using up almost all remaining internal storage! I have already moved and deleted all I can. I have a 32Gb SD card with 23GB available. My tablets internal storage is down to 164MB and it's barely functioning!
While I don't have an answer as to the reason you are unable to move the videos to your SD card, I do have a few possible solutions to your storage problem.
First, if you have a Google Drive account or Dropbox account or One Drive account, I would suggest uploading your videos to one of those cloud storage accounts. Then once the videos have been uploaded, delete the videos off of your tablet.
Second suggestion, you could connect your tablet to a computer, and transfer the videos to the hard drive of your computer, and then delete the videos off of your tablet.
Third, get an OTG cable, connect it to the micro USB port on your tablet, and then connect a Flash Drive to the OTG cable, and once your tablet recognizes the Flash Drive as a storage drive, try transferring the videos to the Flash Drive.
Those are the only options that I can think of right now.
Hopefully someone else with more technical knowledge, than me, can chime in with some additional suggestions as how to fix your problem with the SD Card.