1) Is the sensor looking and cannot find the marks? You may need more light to shine inside (ie: your light source is directly above the printer and dark inside), this is common. The sensor will find two or three marks and not look for the forth mark. Hold a flashlight to the marks as the sensor reads the mark.
2) Your tech may need to adjust the sensitivity of the sensor (voltage adjustment).
3) Is your pinch wheels outside the crop marks? The wheels must be outside the marks (as it was printed).
4) Have you tried another file? Try using one that you have print/cut before, shrink it down to about a 6" size and see if the sensor will find the marks.
5) You will want to leave about a 6" "tail" out the back of you media, this will allow the back
paper sensor to be covered as it performs the reading and cutting.
6) Don't forget to adjust the CROP Adjust, if you've never done a crop cut before the cut most likely will be off - this is NOT the Print Cut Adjust setting you normally adjust.