reason for report file loaded in my Windows Forms application being empty?

What could be the reason for the repor filet I designed in FastReport and loaded in my Windows Forms application being empty? In that file, my query is a simple join with only one int parameter, and no matter what value I call it with, the report returns empty... Please, if anyone has experience with this, tell me... I'm exhausted.

this is my query in fast report designer:

SELECT P.firstName, P.lastName, P.fatherName, P.phoneNumber, P.address ,co.name FROM "ConferenceParticipant" C, "Participants" P , "Conferences" co where P.id = C.participantId and C.conferenceId = @SelectedConferenceId and co.id = C.conferenceId

and my parameter in fastreport is :

SelectedConferenceId