Fazal,
If you check SDF sales doc control in vov8 and check in General Control you will see an Option called "Reference Mandatory" Which is Marked as C (with reference to Sales Order) Which means if you try to Create SDF with reference to Sale order then items will be copied automatically in line Items.So copy control 304 in VTAA is set respectively.
Whereas you are trying to Create SDF wrt Return Order so you'll have set your copy control requirement as303 which will work for both cases either with reference to sales order or Return Order.
Just try it with both scenarios by maintaining requirement as 303 you'll see your items will be copied.
Dear SCN Members please advice if my answer requires any further correction.
Thanks
Zishan