OM Extension not defaulting value on 1st line for FBDI import

I have written an OM extension to default Demand class from customer master to Order line.

Its working fine when order is created from UI.

But when we are importing orders using FBDI, the extension skips 1 line.

e.g. there are 10 SO lines, the Demand class is not defaulted on 1st line but defaulted on the next 9 lines correctly.

The Ext code:

import oracle.apps.scm.doo.common.extensions.ValidationException;

import oracle.apps.scm.doo.common.extensions.Message;

def logger = context.getLogger();  

def lines = header.getAttribute("Lines");

// get attribute to populate in the payload

String submitFlag = header.getAttribute("SubmittedFlag");

Long accountId = header.getAttribute("BillToCustomerIdentifier");