Hi Dave,
Based on your suggestion,for the upgraded version, I looked into the table NB_Store_Model in database and found out that the model ref code was not stored on this table even though I already input the model ref for the product before and it is also displayed on the product detail page. Besides, when I queried this table on another website with new NB_Store installation, the model ref code is stored on this table. I don't know why. Perhaps, I should remove the current NB_Store module and try to re-install again.
Thank you very much for your help.
Regards,
Phuoc