Check list for proper ADO joining:

• Joining a shape file to an external database requires a one-to-one relation.
• ​Identifiers in a coverage must be unique, integer, and never empty.
• ​It is recommended that a coverage be sorted by an identifier (ADOPrimary field) and the same order applied in the external database (ADOSecondary field). This speeds up a join operation dramatically.
• ​If a query is based on an identifier (ADOSecondary) in the external database that is not unique, only the first matching record will be used. We strongly recommend avoiding this situation.
• ​If a join is not possible, joining will turn off automatically. To turn it on again, reapply the ADOJoin, ADOPrimary, or ADOSecondary parameter.