There are two ways that someone can add you as a channel manager and unfortunately is only compatible with one of them.

If someone adds you as a manager to their channel though a "Brand Account" on Google, then when you sign into and click to add your channel, the correct channel will show up. 

However, if someone adds you as a channel manager through the Permissions tab in the Channel Settings within YouTube Studio, then is not compatible.

The reason for this is because we tie into YouTube's API for data and for some reason, YouTube's API only works with channel managers on brand accounts. We hope that they fix it some day.