Depending on how your footage was shot, video can still look blurry or erratic even after it has been stabilized.
For example, if the camera made fast movements while recording at a low shutter speed, stabilized video might still appear blurred.
Large objects moving across the screen can also cause stabilized video to look erratic.
For best results with stabilizing video, record footage using a high shutter speed. Also, use a wider angle with the camera farther away from the subject, so that when your video is stabilized, iMovie can zoom in to the center of your video without cropping out the subject matter.
Note: Some video, such as footage shot with camcorders that use CMOS image sensors, may appear distorted after stabilization. These camcorders often use a technology called a “rolling shutter,” which exposes different parts of the frame at different times until the entire frame is fully exposed. If the camcorder is moved before the entire frame is fully exposed, the resulting image may appear distorted. Applying video stabilization to this video may make this distortion more apparent.