I've never had a lot of lock with longer videos myself. I almost always end up moving them to a computer and uploading from there.
Anyway, there is more than one way to upload a video off hand. So make sure you've tried them all.
The most obvious is to upload it directly from the iMovie app. If that is not working, try saving the video to the Photos app first and uploading it from there. Since they use the same share extension, there might be much difference.
You can also try uploading from the YouTube app. There's an upload button near the top right in your channel view. The video needs to be in the Photos app first.
You might be able to use the website through Safari. I'v never tried to upload a video this way, so you'll have to experiment for yourself. Again, it will need to be in the Photos app.
If none of those work let me know and I see if I can think of anything else.
Of course, having a good wi-fi connection is very important. Also, you may want to disable auto-lock in Settings > General. That should keep the iPad from going to sleep in the middle and maybe stoping the upload.
Good luck.