Hi Wang Yang,
Which programmer are you using, on board KitProg or MiniProg3?
I use on board KitProg2. I keep the board AC OFF for a while and it works OK to program.
Still, I don't know the reason, if you have any idea please share with me.
Could you let me know what the 'AC' refers to?
What's the VDD voltage you selected on SW5 during programming?