Responsive images are not being served

Hi Guys look for a little help if possible. I have been optimizing my pages and I have noticed that responsive images don’t seem to be served although they are being uploaded to the server. I have one page I am concentrating on at the moment, the main header image is 960 wide x 280 high and is around about 136KB in file size, this image has already been compressed before being uploaded. When I view the page on mobile in portrait view I would expect the 500 image to be served which on the webserver is showing around 65KB filesize. If I view the page on mobile and then download the image it is the full 960x280 at the full 136KB file size.

Any ideas what could be wrong? Thanks in advance for any help or cure offered.

PS - I have disabled caching on the webserver and cleared out my browser cache to get a live, true test.

Regards John

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Page I am talking about - Lighting Hire