Hi,
If Your main report is blank,Does it mean that your parentId(Link Field) is NULL?
If its null then there might be a problem.
Let Us Assume that your ParentID is Blank Or Null.
Then if your link field in sub Report say "ChildId" should be Blank or NULL.
so if these two fields that is
the ParentID in Main Report and the ChieldId in sub Report has same data then you will have your report displayed.
Regards
Keval Raj