"Randomizing uploaded headers" acts differently on ie vs chrome & firefox

Dear all,

I am running WordPress 4.3.1 and using Flat Responsive theme.
Site address: http://lparemote.com (A site I am developing for the school I work at).

9 appropriately sized images appear in the “Previously uploaded” section and the “Current Header” is set as “Randomizing uploaded headers”.

I navigated to this section by clicking on:
Dashboard/Appearance/Header/Randomizing uploaded headers

My problem is that on Chrome, Firefox and other browsers, the header changes with each refresh; however, on Internet Explorer, each page seems to have a link with one specific image from the bank. When the home page loads, one specific image is always chosen on IE. Refreshes do not change the header image. This defeats the purpose of choosing the randomized option.

On Firefox and Chrome, page refreshes on my front page yield different header images. On IE, it is the same one all the time.

To assist, the following links are to “Pages”:

http://lparemote.com/policies/
http://lparemote.com/governing-information/

These links go to “Posts”:

http://lparemote.com/drop-everything-and-read/
http://lparemote.com/barcelona-trip-2015/

I have spent a fairly considerable amount of time looking through forums here and elsewhere. I hope someone can tell me what I have missed.

Much appreciated.
Dermot