Calculated Object not returning data on report.

I have 3 objects created in Universe.

Now I want to create one more object which has calculation on there 3 objects.

i.e.  Obj Calc=Obj A + Obj B -Obj C

Object A and C are coming from same table and has NULL values in it.

Object B is coming from another table and has values in it.All 3 objects are of Decimal datatype.

I have defined objects as ISNULL(ObjName ,0.0)

Defined calculation object as      @Select Obj A +@Select Obj B -@Select Obj C.

Everything is Parsed at Universe.

However when I take this Calculated OBj on WebI,it doesnt return anything.

Also when I drag obj A and Obj C individually on report,they return nothing.

Please let me know who will the calculated object will return data on report?

Thanks in advance.

Archana