A couple of responses:
1/ "You already disable the AwoCoupon Free plugin,"
Response: I suspect this may have something to do with it. I tried to un-installed it from extension manager, but the back end Virtuemart component 'Awocoupon virtuemart' stays. Would this have anything to do with it (some bits left over)? How would I be able un-installed it completely?
2/"I have not seen this issue with coupons not staying in the cart for a while. There was an issue with a much older version of Virtuemart, which I posted to Virtuemart forum here:"
Response: as you can see on our development site, this is a site to upgrade to the latest VM 3.0.8.
3/"This seems to be a new problem. I would try disabling the template and using virtuemart default to see if it makes a difference."
Response: the template supplier has done all this and checked thoroughly with the feedback provided in the previous message.
4/ " Also try using a different payment method (standard)."
Response: the standard checkout/payment is working happily. Just the coupons (even a coupon 'testgift' created from VM directly) are not having any effect, and the coupon status at vm coupon stay permanently 'not used'. (feel free to use the credentials i provided in the private email to check the back end.)
I seem to have made some progress by renaming 'com_awocoupon' (although it is disabled and un-installed) from the file manager to force the admin 'Awocoupon Virtuemart' component into 404. I then first time got a coupon working since the Awocoupon Virtuemart install. Will conduct further testing, but at this staging the issue seems to be associated with the Awocoupon Virtuemart's inability to be 'disabled' or 'un-installed'. I still have it under my components(404), not sure how to get rid of it completely/thoroughly?
Also, there is outstanding question (through email): with orders over certain volume coupon, we would like to offer a percentage coupon, say get a 10% discount coupon for orders over $120 but NOT the actual percentage value of the previous order. Is this possible?