YouTube re-encodes uploaded videos to a few different quality settings and will default to playing the lower quality version when you first watch in your browser.
To switch the player to the HD version you must click the cog icon and select the 720p setting:
This could also be due to the fact that initially, Youtube publishes the video at a lower quality, so it's available faster. It then processes the video at a higher quality and re-publishs it later. This process can sometimes take awhile.
In Youtube's own words,
"While this processing happens, your video may appear to be missing higher resolutions for several hours. Once high-resolution processing is finished, higher resolutions will be available on your video."
Please check your video again in about an hour and see if it shows up in higher quality. You can read Youtube's own explanation of the "temporarily lower resolution video" by clicking on the link below:
If it still shows up as low quality after a half hour or so, please check the settings in your app by following the instructions below.
When you first launch the recorder, please choose a minimum of 720p for the size(full screen is the highest resolution):
And when you're ready to publish, please change "Normal" quality to "Higher" for best resolution:
Once you've made sure you have the highest quality settings, you can now upload to YouTube.