Christina,
Did you create this field in SFDC
first? If not, that may be part of the problem.-
If you create the field in SFDC first on both Lead and Contact,
-
then setup Lead Mapping for that field. (under Leads > Fields) That way, SFDC knows to bring in that Lead's field value to the Contact's field.
Marketo will see the new fields and update its database properly. Then you should be able to update things properly.