If the above answers haven't done the job, then you might have the same problem we did - if you don't have any changes on the Salesforce side, then accounts won't sync.
Let's say you did an initial sync, but because your settings for import were incorrect, it only pulled half the records. The bi-directional sync will now check for changed records every 5 minutes from Salesforce. But unless those accounts have a change, they will never be pulled into Marketo. Marketo doesn't look for differences for the whole database, only modifications to the "Date Modified" stamp in SF, and it syncs those.
Make sense? So maybe you had some issues with the first sync, and because those opportunities haven't been modified in SF since, it hasn't changed. It might be worth doing a full sync refresh.
Phil