paper tape punch isnt punching

I bought a paper tape reader/punch on ebay, a DSI NC2400. unfortunately, 2 of the holes arent punching through the mylar tape that came with the unit. does anyone have experience with this unit, or know what might be causing the problem? I know western numerical controls sells refurbished units, but I expect they would charge quite a bit just to replace the punching mechanism. their web page mentions something about "The precision die blocks are interchangeable and field-replaceable in just minutes, no adjustments necessary". so, I'm hoping that means I can buy a new set of punch parts and replace them easily. and hopefully inexpensively. any suggestions welcome, especially if they mean we stop discussing the job market. -Lawrence LeMay