problem linking tables

i can 't add tables to an existing relationship. the identifier of each new table is already existant as foreign key in an existing table called 'etablissement' : when i insert the new table and create the link with table 'etablissement' each table become marked with '*' et when i save i get this message :
"
Error ODBC : [Microsoft][ODBC SQL Server Driver][SQL Server]Conflict between
instruction ALTER TABLE and the constraint COLUMN FOREIGN KEY
'FK_etablissement_delegation'. the conflit happened in database
'maintenance', table 'delegation', column 'id_dele'."

nb fields are identic and nonnulls.The table's field where you'd like to set a foreign key(table 'delegation', column 'id_dele') contains an ID that does not exist in the table's field where you'd like to set a PK.