Newby Question: Loading wav samples

I just purchased a MPC500 so my fifth graders can add push button sound effects to their talent show. I've downloaded several wav. samples from the internet, both using a USB cable and directly by inserting the compact flash card into the media slot on my PC. Either way, they show up on the MPC500.
But when I try to load them, I get the message, "Unknown File Format". EXCEPT, that twice it worked correctly. Any suggestions?

What program are you editing the wavs in? Sound Forge, Pro tools, Audacity? They can be corrupt files or not a wav standard. Convert them if possible.

I haven't edited them at all. I just downloaded them from the internet and transferred them into the memory card. If editing is necessary, that could be the missing step.

just try it. it should help