Kenwood Chef Food processor attachment stops working

Hi, we have a not to old (maybe 5 years?) Kenwood Chef with a food processor attachement. Recently whenever food is put into the food processor the knife stops turning or only turns very very slowly. the discs for grating still work its just the knife part that doesn’t work anymore. Using the Kenwood Chef in its own for kneading etc also works well. So I am pretty confused what the problem could be.