Uploading directly to You Tube from iMovie, or from any third party uploader, can be problematic. You Tube sometimes makes changes to its uploader that iMovie hasn't yet caught up with.
The most reliable way to upload to You Tube is to share out your movie to your desktop and drag it into You Tube's upload window that you get when you sign in to You Tube.
You didn't mention the source of your video clips. If they are from an iPhone 7 or later, they possibly are using the new HEVC codec, that will happen by default unless you change your iPhone video settings to "most compatible". I'm not sure whether You Tube supports HEVC yet, but if your vid is HEVC you can go to You Tube's help feature to check that whether it can be uploaded. I think that sharing it out to your desktop will convert it back to H.264, the old codec. If not, you will need to use a third party converter, like Handbrake, to convert the video to Mp4 or other You Tube supported format.