Hi,
The problem is that we do not do something special in the product page. For all of our pages we include the custom css with the same technical.
So, if with some pages, the css is included and in other, it's not. it is not an HikaShop bug.
Please check your cache extensions or any other extension which could modify the joomla content (like an extension to minimize CSS for example).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.