DLookup does not return correct value

I have a problem opening a form filtered to a particular record. The form is opened by obtaining a values for respondentID, QuestionnaireID and ResponseSessionDate in a dialog. When all selections are made these are used to identify the record in ResponseSession using a DLookup. The combination of values obtained is unique.

Using the dialog I can open the form for some records correctly, whereas for others it does not.

The Dlookup is:

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