To be able to get an intelligent answer You can not rely on my or our mindreading capability.
You must provide lot's more info - so we know where to start.
• What material was used (from what Camera in what video-codec, audio file and photo file format)
• use of iMovie and Mac OS and QuickTime versions (QT is the heart in this process)
• How did You go about in Exporting
• Free space on Start-Up Hard disk (others do not count)
• other devices connected as external or net-work Hard Disks ( formatted as ? )
As You see You leave us out in a waste land of Guessing - if possibly at all
Yours wanting to be of help - Bengt W