Most likely reason for some images not loading until refresh ?

[BlizzardUK]

EDIT : I may have jumped the gun, I just tried the site in a different browser (Chrome) and it seems to be loading okay, browser (Firefox) related then I guess maybe ?

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One thing I have noticed with my new Prestashop 1.6 site is that I can click on a product, or a catagory page, and sometimes the images don't load, but if I refresh the page they load. This doesn't happen everytime but does happen enough to be annoying. Any ideas what it could be or how I diagnose it ? Thanks.

Edited March 9, 2019 by BlizzardUK (see edit history)