What is inside that MP4 wrapper?
Codec & Format information, with 2 links inside to read
-read both links in reply #1 http://forums.adobe.com/thread/1270588
Report back with the codec details of your file, use the programs below... A screen shot works well to SHOW people what you are doing
What is the brand/model of the camera recording these videos, and what camera setting did you use?
Is your Premiere Elements 11 on Windows or Mac.
Important. Do you have the latest version of QuickTime installed on the computer with your Premiere Elements 11? And are you running Premiere Elements 11 as Run As Administrator or from a User Account with Administrative Privileges?
Is your video card driver up to date according to the web site of the manufacturer of the video card?
Would you class your issue as "it never worked before" or "it worked before but not now"?
How would you describe your computer and its resources?
Let us start here and then decide what next.