What version of Premiere Elements are you using and on what computer operating system is it running?
Please check to make sure that keyframing of the Opacity is not taking place.
With information about your Premiere Elements version, I can give you specific details on finding the Opacity Panel expanded
in Properties or Applied Effects Palette to check the status of the Toggle Animation button.
Premiere elements 11 on windows 128 bit
Sure you do not have Window 7 32 or 64 bit?
1. Start out with your clip imported to Premiere Elements 11 Expert workspace Video Track 1 and selected/highlighted.
2. Go to Applied Effects Tab/Applied Effects Palette/Opacity Panel expanded
Make sure the Toggle Animation Button is not highlighted
Move the Opacity slider to the Opacity % that you want
3. Be careful that you do not accidentally move the orange line running through the video up (up or down). That will adjust
the Opacity which is the default for a video clip orange line (line called rubberband)
Please review and consider and then let us know if the above is the information that you seek.