Why won't some videos import properly on Cubase elements 7?

For some reason, some of my videos show up in cubase, but others don’t. Here’s what happens :

I use my iphone 4 to record a video
I convert it to mov at 30 frames per second
I change the name
I try to import it to cubase, and for some videos it works fine, for others it takes an eternity to generate the thumbnail image, and for some videos it doesn’t import at all. It only imports the audio but for the video, it only shows the part, with nothing there. It makes no difference how I import it.

At first I thought it was something to do with the length of the video, because I did one that was 2 mins long at it worked but those that were much shorter didn’t.

But then today I did a 2 min one, and it didn’t work.

I’m totally confused. I’m not doing anything differently for each video. I need to make lots of videos and I don’t want to waste my time doing so only to find that I have to do them again.

Why do some videos import successfully and not others? What do I need to do to make sure that everything works consistently?