I'm trying to join this table (below) to another by matching the column rowNoVFPD to one in another table.

The column VFPDplus1 is assigned using the formula:
Lag(:VFPD, -1).
When I try to do this with JSL the VFPDplus1 column copies across blank, but otherwise correctly.
When I try using the JMP "Join" option in the "Table" toolbar it copies across correctly.
How can I solve this in JSL?
My current join function is written:
Data Table( "markTableSubset" ) << Join(
with( Data Table( "VFPD" ) ),
by matching columns( :FloorVdiv64 = :rowNoVFPD )
);