Importing Stills from iPhoto

I have created an album containing the selection of stills I require in my project. then in iMovie clicked the camera icon, selected the album, selected all the photos and dragged them to the project. Then found that about 10% of the stills did not transfer. Went into iphoto again and tried to add these photos individually. They appear to be dragged, a green vertical bar appears at the location but on releasing the mouse nothing is placed in the project. The same action with previously accepted stills works fine. The unaccepted stills are from the same camera as the accepted ones and have not be enhanced in any way.??