Some MP4 Videos Won't Play

But some do. Here's the story:

In My Videos folder a large majority of the clips have .FLV suffix and play OK. Some of the downloaded videos have the VLC cone displayed and are labled mp4, , and when the thumbnail is pressed the little VLC control box comes up like it is going to play, but it doesn't. I believe all the downloads play through RealPlayer and when attempting to play a just downloaded video an error message (with no details) comes up on the RealPlayer page. I show a folder for Applian FLV and Media Player on the Desktop, not sure where it came from or what it does, but think it was with the last RealPlayer update.

I hope I have explained this clearly enough and with enough detail. And like I said, some videos labled as having the mp4 format play just fine from the download. And all the videos in question played OK on YouTube and appeared to download OK, but simply won't play off the downloaded file. Not a big problem, just keeps me being able to download and store some music I like to frequently listen to.