What do the sizes represent? The area you are using?
once you take out the material the gx-24 automatically senses it and goes to unsetup mode. the size of the material is determined by manually selecting roll or sheet etc. and the optic eye scans the width of the material and sets the machine up that way. nothing has to be done via software.Just a thought....when you switched over to the second roll of material, did you also make sure that the cutting software was aware of the new roll size? I know some software will send the material size from the cutter to your computer if you are using a USB connection, but if it hasn't then this could be something to look into as well. You may have to manually insert your material size into the software so it knows the cutting area.
Do you have your pinch rollers set up under the white areas and as far to the left and to the right as possible?
Also, check to make sure there isnlt something on the underside of your vinyl that is sticking to the pinch rollers.