I started GT GE Mark VI in simple cycle since load dispatch required additional generation in the grid. Started HRSG, after all pending maintenance work. I gave command to GT for steam temperature matching and IGV control the GT exhaust temperature to 400oC. GT control goes from PRESELECTED to PART LOAD. After HRSG diverter damper fully opened, I tried to select the GT to preselected load but failed for sometime. What are the things that restrict in selecting PRESELECTED LOAD from PART LOAD?