Here is a helpful tip whenever you have to put on your ‘CSI’ hat to investigate a SFDC failed sync issue.
The robust integration between Marketo and SFDC is one of Marketo’s most powerful features and it is frustrating when problems arise. Recently I was helping a customer determine why they were unable to successfully insert a Marketo only record into SFDC. We started the investigation by looking at recently created records in Marketo which were inserted into SFDC using the Sync Person to SFDC flow step. When the client checked SFDC they found the newly inserted records. However, when they converted them to a contact a duplicate would magically appear in Marketo. When we looked at the original record in Marketo we noticed it was missing a SFDC Created Date and there was a Failed message in the activity log of the record. The SFDC information wasn’t flowing back to Marketo after the insert. We discovered the SFDC team had recently added a lookup field and the field needed to be populated before the record was inserted into SFDC for the first time. Once we made sure the lookup field was populated prior to the sync it fixed the issue.
Here are some additional resources for learning about the Marketo integration with SFDC.