If you previously hid a particular field from the Marketo profile through field-level security and now you expose it to Marketo, the existing field values will only sync to Marketo when a field on the record is modified and the last modified date changes.
If you want all the values of the field to sync over to Marketo immediately you can have your SFDC admin use the SFDC Data Loader to change the last modified date/system mod stamp for all the records. Another option is to contact Marketo Support have them run the “force re-sync object tool.”
Note: If you are changing the last modified date for a lot of records it may impact sync speed, so you should consider doing it overnight or over the weekend.
Additional resources for learning about the Marketo integration with SFDC.