It sounds like you are putting them on the card the wrong way. At least the wrong way for this to work.
I am assuming, of course, that you are taking the card out of the phone, connecting it to the computer, copying the data, and then putting it back in the phone. If that is not the case, please do let me know.
Now, if that is what you are doing, you really shouldn't be removing the card from the phone to add data. The simplest way to make it work would be to leave the SD card in the phone, connect the phone to the computer via USB cable, and then copy the files over that way. This would avoid any conflicts as the card would stay formatted for the phone.
If the files won't move over to the card when done this way, then there is something going on with the files.
Also, be sure that they are files your phone can actually read/view. Just because a computer can doesn't mean a cell can.
Lastly, make sure that your phone is cable of handling a SD card of that size (size as in storage amount, not physical size). If the card holds more than the specs say is the most the phone can handle that will also cause you problems.