about 52840 (pca10056) program bootloader

I am programming with SDK15.0. Then I use nRFgo studio to download the merged program to the DK board. But it reported a validation failure. So I tried to download softdevice and bootloader separately to find out the problem. It still reporting an error. But Everything works fine when I use Keil download. Could you tell me where the problem is?

Thanks

June6