Any chance that the coaxial power connector's pin is too small for the mating plug? There a connectors with 2mm and 2.5mm pins.
Do the ribbons always have power on them? If there are traces on the LED ribbons for +12V and common (or whatever they call it), that would be the proper place to check to see if the supply voltage is getting out to the string. You might need to scratch away some insulating coating to make contact.
(I don't have any multicolor ribbons and controllers .... yet.)