Make sure your mapping of segments are perfect(IData Values).
And Instead of giving nsRecord,try with passing the ffschema param(EDIFFSchema.X12.V4010:T835)of convertToString,This should work.
If this problem still exists,then problem might be with mapping itself.
(Above mentioned schema will be stored in EDIforTN Package.)
HTH.
#edi#webMethods#Integration-Server-and-ESB