It sounds like you are either using non-standard video (such as video from a still camera or pocket camcorder) or your project settings do not match your footage.
What model and make of camcorder is your video coming from and how did you get it into your computer?
Welcome to the forum.
Can you post a screen-cap showing the shadow? I have some ideas, and that screen-cap will tell me what I need to do to help you.
I'd also look at this ARTICLE, as it has plenty of tips on what info will so useful to us, trying to remotely troubleshoot your problem.