Product not Found

-- HikaShop version -- : 3.1.1
-- Joomla version -- : 3.7.4
-- PHP version -- : 7.0
-- Browser(s) name and version -- : Chrome 59.0.3071.115
-- Error-message(debug-mod must be tuned on) -- : Product not Found

Hi,

The listed URL shows 9 products, they are all valid products but the top 3 products give the message 'Product not found" when I try to go into the detail view. The rest seem fine. Before I upgraded Hikashop (2.6.4 to 3.1.1) they all worked. I have tried clearing the Canonical URL's from the products, Checking the database and clearing caches but no change.

So is there anything else I can check? I don't know if this is an isolated issue or if other products are affected.

Regards
Tim