Hello,
If a product have variants, you can assign the pack with the main product and it will also include all the product's variants.
But if you just want to target specific variants, you need to perform the assignation to the variant level.
Afterwards, the "attach serial" plugin is working on the database level ; if the data is in the table order_product, the plugin will be able to display it.
Does the information stored in your database ? Can you see it in the backend when you display the order ?
I could be it's related to a bug in HikaShop 3 (we made some patchs in the past months).
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.