Are you shooting the videos vertical (tall)?
iMovie edits video for TVs, so can only handle videos that are wide, and can only export wide.
This is a limitation that is not specifically said in the literature as far as I can tell.
If you bought the app recently, you can ask for a refund:
- First, open iTunes and select the iTunes Store link in the left-hand column
- Near the top right corner, click the arrow next to your user name (email address) and then selectAccount

About half way down the screen, click the Purchase History button

In the bottom portion of the screen, you will see your App Store purchase history -- click the Report a Problem button

Locate the iTunes invoice with the application you would like a refund for, and click the Report a Problem link

Fill out the form that follows and be sure to be as detailed as possible -- when finished, click Next

If your reason for requesting a refund is valid, Apple should respond within a few days and process your refund within a week.