saving problem from D2K Form

D2k
hi everyone!
i created a Form using Developer 2k & table from Oracle 8i

this Form has 40 columns linked with 25 tables.

some of the columns have attached LOV so user can select value from LOV

i have given 2 SAVE button, one in the Form & other in Menu Option Bar

but problem is that after feeding values (some value selected from LOV) when I click the Form's SAVE button it's Alerting Message "Field Value cannot be null" & cursor Jumps to First LOV attached Column(VALUE ALREADY FEEDED OR SELECTED FROM LOV) & can not be saved the record but when i click SAVE button from Option Menu Bar then it's running fine but users want SAVE button on the Form.
(same code is written in the TRIGGER on both SAVE button)
what 'll be the solution ?
any idea? plz guide

R