For some reason, Itunes won't add my video?

I have a video on my computer and want it on my iphone. It's.MP4 format, and I looked at it properties. I unchecked the "Read Only" box, but iTunes still wouldn't add it. I tried opening it to play in iTunes, and it wouldn't even open the video. I tried to add it by going to "File" as well as dragging it.

Not all iTunes video can be transferred to iPhone, If the video resolution or video bit rate beyond iPhone limitation, you can't upload them to iPhone. For example, some iTunes purchased movie are High definition movie, the resolution is too high to play on iPhone, but you can highlight the problem video in iTunes and choose Create iPod or iPhone version from the Advanced drop down menu. When the conversion process has completed, sync the new version to your iPhone. Fore more info, here is a guide about how to convert video to iPhone/iPod. Hope it helps. Contact me if you need further assistance.