The reason might be because you have No Caching setup and it might be messing with your headers.
HOW IT WORKS
The No Caching tool prevents caching by manipulating the HTTP headers that control the caching of responses. The If-Modified-Since and If-None-Match headers are removed from requests, Pragma: no-cache and Cache-control: no-cache are added. The Expires, Last-Modified and ETag headers are removed from responses and Expires: 0 and Cache-Control: no-cache are added.