Hi,
This looks like a caching issue on your website.
Please make sure that the caching option of the joomla configuration is turned off, that the cache plugin is disabled, that if your template has some caching options, that they are disabled, and if you use a CDN on your server, that you deactivate it, and it should then work properly.
Regarding the discount 'coupon' that you setup, if it applies on all the order and not just the products, it's probably that the "Coupon percentage applies to product only" option of your coupon is not turned on.
Regarding the fact that the coupon doesn't work when you set a category in it, it's probably that the "Including sub categories" option is not turned on in your coupon.