Your field has displayed on multiple levels of the Field List tree, and you have selected from a level that does not correspond to the line item section of the report.
Drill to the next level of the field list tree so that the field dragged into the report has the correct relationship set up. By selecting from deeper in the tree, the SQL behind the report can "see" the individual values for
each line item and returns them, instead retrieving the first value and repeating that value on each line.