LM5170EVM-BIDIR: LM5170EVM can only work in boost mode when HV = 42V, and LV = 21V. Buck mode doesn't work and just outputs zero current

I am using a LM5170EVM. When it is configured as High Voltage (HV)  = 42V, and Low Voltage (LV) side = 21V, it can only work as boost mode, and the buck mode will just output zero current. 

When the EVM is configured as HV = 42, and LV = 10.5, it can work in both buck and boost modes, and there is no problem. 

Can anybody help to solve the problem? Many thanks.