Hi Gabriela! There are two possible reasons why the viewed product event wasn’t tracked for those other 3 sites: 
1. The viewed product tracking wasn’t installed properly. Make sure to add the tracking code following these instruction: https://help.klaviyo.com/hc/en-us/articles/115005076767-Guide-to-Klaviyo-Web-Tracking#viewed-product-tracking3 

2. The person wasn’t “cookied” for that particular site. Just because they are tracked for one site doesn’t mean they are tracked for all the others. You can learn more about cookies here: https://help.klaviyo.com/hc/en-us/articles/360034666712-About-Cookies-in-Klaviyo 

Hi Elise, 

Thanks for your reply! About these possible reasons:

1. I checked tested the web tracking in all websites, and they are working.
2. The person used the same browser and the same device when checking all four websites. Do you think that even using the same browser, the person wasn't "cookied"?

Many thanks!

Hey Gabriela! Yes, that’s correct -- I think this person was only cookied for one of the sites. Cookie-ing happens on a per-account basis, so just because they’re cookied on one site doesn’t mean they’re cookied for the other three.