This can occur if...:

...you are joining two primary keys. A 1:∞ relationship requires that one key only is a primary key. Check to see that you are joining the correct fields.

...you are joining a primary and a foreign key and the foreign key is specified as 'no duplicates' in the Index property for that field in the table.

(a) If the foreign key can have duplicate values in the table (this is the normal situation) then change the Index property to show this.

(b) If the foreign key really is a unique value in the table then check to see if you need to use a separate primary key for this table; perhaps you can use the primary key from the main table in both tables (as in point 1 above).

You should also check your design carefully. Sometimes a 1:1 relationship is correct (e.g. to save space in the database for information that that doesn't apply to all records, or to put sensitive data in a separate table in order to restrict access). But you need to be able to justify each 1:1 relationship that you have, so that you know that it is correct.