lookup value is updating in table

Hi all , 

Using two tables in form datasource HeaderTable and LineTable  - *DOCUMENT NUM* is common in both table and in design i m using groups 

first group i added document num , cust account and custname 

second group added all the line table fields 

in lookup filtering the document number means related document number against line table values will show in line table fields . 

here i m facing the problem . once i enter the values in line table and moving to next document number it was updating previous document number to the current one which i selected .. 

desing which i created above

figure 2 is showing the lookup values total 6 entries .. currently i m in DOC-ASAP000016 then next image i  m moving to DOC-ASAP000013 

After moving to next document number it was updating in the place of DOC-ASAP00016 to DOC-ASAP00013 .. 

how to solve this issue .. any one suggest me the solution for this ....