What triggers Marketo to sync with Salesforce?

For records already synced between Marketo and Salesforce: Marketo will sync periodically all of these.  It looks for a change on the System ModStamp to identify records with some changes since the last sync, and then updates these records.

This sounds like a specific issue you have with the sync.  I'd recommend creating a support ticket

Hi Slalom,

Steven is correct that previously synced records will update automatically. Perhaps there is an issue with the permissions granted to the SFDC Marketo User because what you say should not happen at all.

If a Lead comes into Marketo as an entirely new record, then it WILL NOT SYNC until you explicitly or implicitly ask Marketo to sync that record toSFDC:

• add to an SFDC Campaign
• change SFDC Campaign Member Status, thus implicitly requesting an Add to SFDC Campaign
• Call Sync to Salesforce
• becomes a Member of a Program with an SFDC Campaign connection
• merged with a previously synced record

If a Lead is created in SFDC, (or Contact, etc), then Marketo will pick this up at the next check in.