While we all have MacBooks in this forum not all of us use iMovie. There's an iMovie Support Community where everybody uses iMovie. You should also post this question there to increase your chances of getting an answer. https://discussions.apple.com/community/ilife/imovie
Drag and drop has a special meaning in iMovie.
In your case, try using FILE/IMPORT MOVIE and navigate to your clip.
If that does not work, open your clip in QuickTIme Player and then press Command-I to open the Inspector.
Tell us the dimensions of your clip and everything listed next to the word Format: