Why won't some videos play on my new iPhone 4s?

I've put some videos I took from my old phone onto my iTunes then tried to sync and then wouldn't go on there? Why? And is there anything a can do to change it and make them go on!

1st I assume you had an iPhone before. If not you need to check that the videos are mp4 format. If they are not, they will not play on ur idevice. To check right click on the video and click on properties. You can get free converters online with a simple google.

Try finding your phones format then convert the video to that format/

Itunes will convert any video to the right format.