1. Login to your Thematic account.
2. Click on your profile photo and select the gear icon.
3. Select the Linked Accounts tab.
4. Click the Edit Linked Accounts button and click the Verify More Channels button to connect more YouTube channels to your Thematic account. 
5. Follow the steps on the pop-up window to connect your YouTube channel. You will be asked to login with a Google account and then select your Brand Accounts / Channels.
6. You can remove a connected channel by clicking the "x" button next to the channel, or click Replace to replace the connected channel with a different one.
7. Click the Save Linked Accounts button to save the changes.
8. All of your channels should now be listed.

Please repeat this process to connect all of the channels where you are actively uploading content.