It is most likely a permission setting in Salesforce or a validation rule. Marketo will not check whether fields are populated when attempting a sync to SFDC.
Jep, thanks. Rules in SFDC, Marketo, or both? If in SFDC, which Object? Leads or Marketo API package? We don't have any validation rules set up for Leads.
SFDC only. Since you mentioned the field Job Title and it sounds like you were syncing a new Lead, it is probably on the Lead object. Job Title is a standard SFDC field, so it's not related to the Marketo managed package. Is it maybe a required field?