About Youtube channels and YouTube API v3

I was wondering why is that that some channels return an array in the following code and other don't? I'm trying to make a video gallery but the specific channel won't return me that array that has contentDetails. I also have access to the youtube account if I need to change any settings in there to allow this code to work. I also already have api credentials and added API v3 to the project.

$(document).ready(function() {    $.get("https://www.googleapis.com/youtube/v3/channels", {            part: 'contentDetails',            forUsername: channelName,            key: 'myKeyvalue'},            function(data) {                console.log(data);                //upload_id = data.items[0].contentDetails.relatedPlaylists.uploads;            }    )}

Thanks in advance.

The data object should contain an array.