Rexel AUto 650+ shredder?

we have a rexel auto 650+ shredder which we have only had about a month it has been working fine until today, when you manually feed the shredder it used to start and stop automatically when you put the paper in, but now you have to press start to start it and stop to stop where it used to stop by itself once paper had shredded, i have checked instruction booklet but no clues there