Integrations - Historical data not syncing

Hi folks, 

I am new to using Klaviyo and trying to integrate it into our CRM, Salesforce. I followed the steps and the status is showing as “Enabled” so apparently it was integrated, however when I click on “Data” and try to sync it, it won’t start. 

I had a theory that maybe it wasn’t syncing because it already completed the sync in the first place when I connected to the CRM however when I checked in the metrcis “Became a Lead” it showed that the last sync was a year ago. 

Can someone please help me understand if there is anything I can do to fix this? Your help is greatly appreciated!