You have recently added a new field to a SFDC Custom Object which is synced to Marketo, but you do not see that new field available on the object in Marketo.
To have the new fields pulled over to Marketo, disable your SFDC global sync within Marketo and then refresh schema for custom objects. This should pull over the new fields on that custom object to Marketo.
Once the schema is refreshed, please make sure that the global sync is turned back on. We would also suggest that you refer the below document which will guide you on the above steps. However, the below document is meant for first custom object sync. The same steps need to follow up for the existing object sync as well.
Who This Solution Applies To
Customers integrated with Salesforce