Shed paint

I have a garden shed which is nearly 9 years old and I paint it with cupranol every 2 years.
I painted it in September 2012 and it looks perfect except for 2 of the planks and most of the cupranol has come off.
I have asked in 2 shops and they have both said the cupranol is not taking to those 2 particular pieces of wood for some reason.
The obvious solution is to re paint those 2 planks of wood with cupranol now but is there anything else I can do to stop the cupranol coming off in under a year?.

Frank