Here's a great video that shows you how to load a roll of paper and then "align" the paper.
YOUTUBE VIDEO
The alignment is to make sure the paper is square in the plotter...and the sensors will check when you are done aligning to make sure you did it right, if you didn't it will keep kicking the out the paper load. The only other thing that can affect this is the edge detection from the same sensor...if you get an edge detection error, the light that shines red or green on the black rubber roller is probably dirty and you can reach under and clean it with a cotton swab soaked in rubbing alcohol...do this while the machine is turned off.
YOUTUBE VIDEO
The alignment is to make sure the paper is square in the plotter...and the sensors will check when you are done aligning to make sure you did it right, if you didn't it will keep kicking the out the paper load. The only other thing that can affect this is the edge detection from the same sensor...if you get an edge detection error, the light that shines red or green on the black rubber roller is probably dirty and you can reach under and clean it with a cotton swab soaked in rubbing alcohol...do this while the machine is turned off.