iMovie cant import files from camera roll iPhone

I took some videos and it shows on my camera roll. Im sure the videos are on the camera roll.
But when I opened iMovie and pressed the import button, only videos from yesterday was shown. Newer videos are missing.
I have edited quite a lot of clips using iMovie on iPhone before, some projects also showed missing files (with the yellow triangle) but these videos are also shown on the camera roll.
Do you know how to solve this problem? Thanks!
Windows 7 iOS 4