It is unlikely a 6V, 14W solar panel has sufficient power to charge your power bank.
14 W is a maximum under ideal conditions and very likely an exaggeration by the vendor. A solar panel will have an IV curve. 6V is a nominal voltage and actual Voc will vary widely depending on irradiance and load.
You need to use a shunt resistor to also measure the solar panel's current while measuring the output voltage.
You do not say how you know whether the battery pack is being charged or not.
With your solar panel, if it has sufficient power to charge at all, it may take days of bright sunshine to charge the battery pack. Do not assume the solar panel is providing 14 watts of power, measure it.
A solar source is typically connected to an MPPT rather than a DC-DC converter.
A solar panel is used for practical purposes only where there is no utility power available. It takes many (e.g. 20) years of use to recoup the cost in electricity savings if ever. The cost analysis is not simply electricity cost ÷ (panel watts x time). Usually the cost of the batteries that need to be replaced every few years prohibits any cost savings.