A couple of other things to consider when you upload:
- Every platform is going to recompress your video again for streaming. Some will do this more than others (Facebook is pretty bad, for example.)
- When you first upload to YouTube, it will encode the lowest quality streams first. So you may actually be watching your video at 360p rather than 1080p if you don't wait long enough for it to display the higher resolution versions.
If there is something up with your export settings then you'd need to provide more information, and a screenshot is helpful, per Ann's request.