We created a field in Marketo that was not synced to SFDC. At a later date, we decided that we did want it to sync to SFDC, so we created a custom field in SFDC on the Lead and the Contact, but rather then sync to the existing Marketo field, the system created a new Marketo custom field, so the information is still not syncing correctly to SFDC. The fields do not show up in the Marketo Field Mapping, so I am unable to change them.
This is expected. Fields created in Salesforce do not automatically map to existing Marketo fields. Please submit a support ticket and ask for the fields to be merged together.