Burden is on the person who seeks an amendment after commencement of the trial to show that inspite of due diligence, such an amendment could not have been sought earlier.

Civil Procedure Code, Order 6, Rule 17 - When an application for amendment of pleadings is filed after commencement of trial, whether or not the opposite party has raised any specific objection, before allowing the application - It is the duty of the Court to satisfy itself that the applicant, inspite of due diligence, could not have filed the application at an earlier stage - Reason is that the jurisdiction of the Court to allow the application is taken away unless it comes to the conclusion that, inspite of due diligence, the party could not have raised the matter before commencement of the trial - Burden is on the person who seeks an amendment after commencement of the trial to show that inspite of due diligence, such an amendment could not have been sought earlier.