You can change your channel name upto 3 times in 90 days. If you've changed it recently, you might have to wait a bit longer. Here's more info: https://goo.gl/24V4vC . Let us know if this isn't the case and we'll look into it.
Please send a feedback report directly from YouTube with these steps: https://goo.gl/WdI0BK . While every individual report may not get a direct response, we do review everything and use the details to investigate and fix issues. Thanks!