jQuery setRequestHeader Authorization

I'm trying to authorise an AJAX query based on this code:

...
beforeSend: function(xhr) { xhr.setRequestHeader('Authorization', 'BEARER ' + access_token); },
...

All work fine so far! But there are multiple ajax requests and the request header / the access token is not send each time (Every second or third time).

The first Request-Header looks like this:

• Accept application/json, text/javascript, /; q=0.01
• Referer URL.com
• X-Requested-With XMLHttpRequest
• User-Agent Mozilla/5.0 (Macintosh; Intel Mac OS...
• Authorization BEARER 00000000000

all work fine! But the next request:

• application/json, text/javascript, /; q=0.01
• Accept-Language en-us
• Accept-Encoding gzip, deflate
• Referer URL.com
• User-Agent Mozilla/5.0 (Macintosh; Intel Mac OS...
• X-Requested-With XMLHttpRequest

Does anybody know whats going on here?