Imovie Picture in Picture- won't move and can't resize

I have a macbook pro, with the latest OSX Yosemite installed. My imovie is at version 10.0.9 . I simply have to put a videoclip over the top of another videoclip- for a project i am working on- i have done this many times before, so i am unsure why it is happening now. When i try to resize or move the picture in picture, imovie simply does nothing? i was wondering if anyone could help me with this