LTM8055 Demo DC2017A can't output 12V when input is 5V

hello all,

    I have a LTM8055 Demo DC2017A, the LTM8055 datasheet shows input can be +5V, but when I add +5V to DC2017A input, the output is only 1.65V; when I rise the input voltage above 5.7V, the output is 12V corrently. 

    where is the problem? Could anybody help me?

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