Problem Programming the AT89S52 with Willar SP200S

[pushycat]

I have a commercially and locally available clone of Willar SP200S. I was confortable with it untill i got complaints from my customers when i sent them the programmed microcontrollers, they complained those chips just doesnt work. I got the chips back and reprogrammed but experienced the same problems. I have a stock of more than 150 Nos in my stock so naturally suspecting these is out of the way. So inserted the same chips in a bread board and programmed it through USBasp (modified to program 89sxx). The same chips are working fine. With my curiosity i suspected and measured the Vpp of the programmer at the zif socket which is found to be in the specified range suggested in the schematic. Did anyone experience the same problem ?..please share your experiences and valuable sugessions.
Regards
Pushy