We have a large number of contacts in HubSpot (~5600) that don't appear to be syncing with SFDC at all, despite these email addresses also living in Salesforce (as either Leads or Contacts). These contacts meet our inclusion criteria for syncing. I don't know why HubSpot and SFDC aren't matching up the records and syncing them. In some cases they used to sync but aren't anymore, for an unknown reason. Is there a way for me to force sync this list of contacts to SFDC, even if it means creating dupes in SFDC (which I could merge after the fact)?
You can manually sync contacts or lists by following those steps.
If after that the issue is not resolved I would recommend connecting with HubSpot Technical Support, as Support is included in your subscription and they will be able to provide real-time assistance for this matter, where you are able to share screenshots and further information about the matter you mentioned here.