What is the DC-DC converter your using, and what is the application?
Every DC-DC converter has "soft-start" to limit/control the rate at which the output comes up. This is done by clamping the error amp output to the voltage on the "soft-start capacitor", so the duty cycle will be limited as it starts and along with it the input current. If the unit has a "large" bulk input capacitor, that could be your issue. Other limiting circuits use a MOSFET with parallel resistor in series with the input return leg and a RC on the gate. When the unit is turned on, the resistor limits the current and after the capacitor is charged to a certain level it turns on the MOSFET shorting out the resistor.