On a few YouTube videos, when I load them up on my iPhone, I get an error stating:
"Not available on this platform"
Can anyone explain to me what is the issue with these videos that they can't be seen on the iPhone (is it due to size, etc?)
While uploading videos, the author has an option to enable viewing on mobile platforms. If the author's prevented viewing on mobile platforms, then you'll see this error message.
The error message also comes if the video contains some copyrighted media and the copyright holder has decided to not let the video be viewable on mobile platforms.