sd card from ebay???
I had this same problem - tried everything, was it incompatibility, missing codec or WHAT? And why would it play a video once, but after moving the video from the card to the tab, adding another video to the card, and moving the 1st video back from the tab to the card - it no longer played?
Much grief, then found out that there are many sellers on ebay who sell FAKE memory cards - with only 2GB or so of memory, but they are set to show that there is more memory, and that files are being copied, and that the files are there and the size, and the free memory remaining, but the files themselves are not there after the first GB or two.
Google FAKE SD CARDS and find the h2test2.exe to check your card.
The test showed that I have only 1.8 GB on my card - and (no surprise) when I took the music off, I could actually add 2 videos totally 1.5GB and play both of them, not just the first one!
I'm awaiting a refund from the seller, not sure how ebay will respond.