Syncing Leads to Fetch Lead Owners

I have  several leads in my Marketo database that don't have an assigned lead owner - most all of these leads are synced over to Salesforce automatically when they enter Marketo, and we have auto-assignment rules based on country. But sometimes, they don't sync over correctly.

I ran a smart campaign to sync those leads that aren't synced over yet so they can get an assigned lead owner, but for some reason the assignment rules aren't recognized on the Marketo side, even though I input "Use Auto-Assignment Rules" when I sync the leads over.

It will assign the default lead owner to the lead before reading the country value, is my guess.

Does anyone have any suggestions about how to approach this and correct it? My default lead owner has way too many leads in his queue and they should be assigned to our other lead owners.