If you have this mapped to a contact field, the value will only appear when it is a shared option for both the lead and contact picklists. I'd check there first.
If Kenny's tip is not the solution then look into record types, that may control which values are in fact usable.
Usually the picklist value will only appear if there is at least one lead with such a value. Marketo does not completely 'understand" picklists from SFDC. Yes, it will warn you if the SFDC picklist changes, but it's not inherently part of the database.