SSRS lookup returning blank or skipping rows even though data is available in those fields

I have lookup expression to compare two datasets, the lookup is missing certain rows. Some rows in Totaldownload in DataSetCount have value of that data , but lookup is skipping. Some rows are lookup correctly why some were skipped. I am not sure if anyone has encountered this problem before where lookup is just returning blank to some rows in the column. please see image below ….

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 =IIF(IsNothing(
Lookup(Cstr(Fields!REG.Value) & Format(CDate(Fields!FL_DATE.Value), "yyyy-MM-dd"), Cstr(Fields!REGISTER.Value) & Format(Cdate(Fields!Flightdate.Value), "yyyy-MM-dd"), Fields!TOTALDOWNLOAD.Value, "DataSetCount")), "Missing", 
Lookup(Cstr(Fields!REG.Value) & Format(CDate(Fields!FL_DATE.Value), "yyyy-MM-dd"), Cstr(Fields!REGISTER.Value) & Format(Cdate(Fields!Flightdate.Value), "yyyy-MM-dd"), Fields!TOTALDOWNLOAD.Value, "DataSetCount"))

SSRS lookup returning blank or skipping rows even though data is available in those fields